Set Bundler.settings[:ssl_ca_cert] to download gems

When `bundle install` connects with a certification (CA) to a private RubyGems
HTTPS server emulated by WEBrick, the connection to the
https://localhost:18443/versions succeeded, but the connection to download gems
failed with the following error.

```
+ /home/jaruga/var/git/ruby/rubygems/bin/bundle config set --local ssl_ca_cert /home/jaruga/git/report-bundler-bundle-config-set-ssl_ca_cert/tmp/client/ssl/ca.crt
...
+ /home/jaruga/var/git/ruby/rubygems/bin/bundle install -V
Running `bundle install --verbose` with bundler 4.1.0.dev
Resolving dependencies because there's no lockfile
HTTP GET https://localhost:18443/versions
HTTP 206 Partial Content https://localhost:18443/versions
HTTP GET https://localhost:18443/versions
HTTP 200 OK https://localhost:18443/versions
Fetching gem metadata from https://localhost:18443/
Looking up gems ["hello"]
Resolving dependencies...
Using bundler 4.1.0.dev
1:  bundler (4.1.0.dev) from /home/jaruga/var/git/ruby/rubygems/bundler/bundler.gemspec
Fetching hello 0.1.0
Retrying download gem from https://localhost:18443/ due to error (2/4): Gem::RemoteFetcher::FetchError SSL_connect returned=1 errno=0 peeraddr=127.0.0.1:18443 state=error: certificate verify failed (unable to get local issuer certificate) (https://localhost:18443/gems/hello-0.1.0.gem)
Sleeping for 1.22 seconds before retry
Retrying download gem from https://localhost:18443/ due to error (3/4): Gem::RemoteFetcher::FetchError SSL_connect returned=1 errno=0 peeraddr=127.0.0.1:18443 state=error: certificate verify failed (unable to get local issuer certificate) (https://localhost:18443/gems/hello-0.1.0.gem)
Sleeping for 2.26 seconds before retry
Retrying download gem from https://localhost:18443/ due to error (4/4): Gem::RemoteFetcher::FetchError SSL_connect returned=1 errno=0 peeraddr=127.0.0.1:18443 state=error: certificate verify failed (unable to get local issuer certificate) (https://localhost:18443/gems/hello-0.1.0.gem)
Sleeping for 4.02 seconds before retry
Bundler::InstallError: Bundler::HTTPError: Could not download gem from https://localhost:18443/ due to underlying error <SSL_connect returned=1 errno=0 peeraddr=127.0.0.1:18443 state=error: certificate verify failed (unable to get local issuer certificate) (https://localhost:18443/gems/hello-0.1.0.gem)>
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/rubygems_integration.rb:406:in 'Bundler::RubygemsIntegration#download_gem'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/source/rubygems.rb:531:in 'block in Bundler::Source::Rubygems#download_gem'
  /home/jaruga/.local/ruby-4.1.0-debug-3ef48ef9c8-openssl-4.1.0-7194354488/lib/ruby/4.1.0+1/rubygems.rb:1068:in 'Gem.time'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/source/rubygems.rb:530:in 'Bundler::Source::Rubygems#download_gem'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/source/rubygems.rb:459:in 'Bundler::Source::Rubygems#fetch_gem'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/source/rubygems.rb:443:in 'Bundler::Source::Rubygems#fetch_gem_if_possible'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/source/rubygems.rb:575:in 'Bundler::Source::Rubygems#rubygems_gem_installer'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/source/rubygems.rb:184:in 'Bundler::Source::Rubygems#download'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/installer/gem_installer.rb:29:in 'Bundler::GemInstaller#download'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/installer/parallel_installer.rb:148:in 'Bundler::ParallelInstaller#do_download'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/installer/parallel_installer.rb:132:in 'block in Bundler::ParallelInstaller#worker_pool'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/worker.rb:70:in 'Bundler::Worker#apply_func'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/worker.rb:65:in 'block in Bundler::Worker#process_queue'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/worker.rb:56:in 'Kernel#loop'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/worker.rb:56:in 'Bundler::Worker#process_queue'
  /home/jaruga/var/git/ruby/rubygems/bundler/lib/bundler/worker.rb:98:in 'block (2 levels) in Bundler::Worker#create_threads'
...
```

`Bundler::Fetcher` creates the connection object by the `#connection` calling
`#bundler_cert_store` storing `Bundler.settings[:ssl_ca_cert]` in the following
part. It is used in some parts.

bundler/lib/bundler/fetcher.rb
```
    def bundler_cert_store
      ...
      ssl_ca_cert = Bundler.settings[:ssl_ca_cert] ||
                    (Gem.configuration.ssl_ca_cert if
                      Gem.configuration.respond_to?(:ssl_ca_cert))
      ...
    end
```

However in the case of downloading gems in Bundler,
Bundler calls `Bundler::Source::Rubygems#download_gem`
calling `Bundler::Fetcher#gem_remote_fetcher`
calling `Bundler::Fetcher::GemRemoteFetcher`
extending `Gem::RemoteFetcher` managing `@cert_files` for RubyGems.

Therefore, the `Bundler::Fetcher::GemRemoteFetcher` needs to update the `@cert_files`
by adding the value of the `Bundler.settings[:ssl_ca_cert]`.

As in the process of downloading gems,
`Gem::Request.configure_connection_for_https` is called, and it gets
`Gem.configuration.ssl_ca_cert`,
we don't need to add the value in the `Bundler::Fetcher::GemRemoteFetcher`.

According to the following logic, `@cert_files` is always not `nil`,
as `Dir.glob(patterns` returns Array. We don't need to consider the `nil` case.

lib/rubygems/remote_fetcher.rb
```
  def initialize(proxy = nil, dns = nil, headers = {})
    ...
    @cert_files = Gem::Request.get_cert_files
    ...
  end
```

lib/rubygems/request.rb
```
  def self.get_cert_files
    pattern = File.expand_path("./ssl_certs/*/*.pem", __dir__)
    Dir.glob(pattern)
  end
```

Add unit tests for `Bundler::Fetcher::GemRemoteFetcher#initialize`.

Author: junaruga

bundler/lib/bundler/fetcher/gem_remote_fetcher.rb

@@ -9,6 +9,8 @@ def initialize(*)
         super
 
         @pool_size = Bundler.settings.installation_parallelization
+        ssl_ca_cert = Bundler.settings[:ssl_ca_cert]
+        @cert_files << ssl_ca_cert if ssl_ca_cert
       end
 
       def request(*args)

spec/bundler/fetcher/gem_remote_fetcher_spec.rb

@@ -6,6 +6,36 @@
 require "bundler/vendored_persistent.rb"
 
 RSpec.describe Bundler::Fetcher::GemRemoteFetcher do
+  describe "#initialize" do
+    context "when ssl_ca_cert setting is not set" do
+      before do
+        allow(Bundler.settings).to receive(:[]).and_call_original
+        allow(Bundler.settings).to receive(:[]).with(:ssl_ca_cert).and_return(nil)
+      end
+
+      it "does not append the setting to @cert_files" do
+        cert_files = subject.instance_variable_get(:@cert_files)
+        gem_cert_files = Gem::Request.get_cert_files
+        expect(cert_files).to eq(gem_cert_files)
+      end
+    end
+
+    context "when ssl_ca_cert setting is set" do
+      let(:ca_cert_path) { "/path/to/ca_cert" }
+
+      before do
+        allow(Bundler.settings).to receive(:[]).and_call_original
+        allow(Bundler.settings).to receive(:[]).with(:ssl_ca_cert).and_return(ca_cert_path)
+      end
+
+      it "appends the setting to @cert_files" do
+        cert_files = subject.instance_variable_get(:@cert_files)
+        gem_cert_files = Gem::Request.get_cert_files
+        expect(cert_files).to eq(gem_cert_files + [ca_cert_path])
+      end
+    end
+  end
+
   describe "Parallel download" do
     it "download using multiple connections from the pool" do
       unless Bundler.rubygems.provides?(">= 4.0.0.dev")