rubygems: Fix Gem::Request for PQC support, adding integration connection tests

Added PQC server/client connection integration tests.
As test_pqc_ssl_client_cert_auth_connection failed with the following error
due to hardcoded `OpenSSL::PKey::RSA.new` in
`Gem::Request.configure_connection_for_https`, fixed it to support ML-DSA ssl_client_cert.

```
Error: test_pqc_ssl_client_cert_auth_connection(TestGemRemoteFetcherLocalSSLServer): OpenSSL::PKey::PKeyError: incorrect pkey type: UNDEF
/home/jaruga/.local/ruby-4.1.0-debug-3ef48ef9c8-openssl-4.1.0-7194354488/lib/ruby/4.1.0+1/openssl/pkey.rb:394:in 'OpenSSL::PKey::RSA#initialize'
/home/jaruga/.local/ruby-4.1.0-debug-3ef48ef9c8-openssl-4.1.0-7194354488/lib/ruby/4.1.0+1/openssl/pkey.rb:394:in 'Class#new'
/home/jaruga/.local/ruby-4.1.0-debug-3ef48ef9c8-openssl-4.1.0-7194354488/lib/ruby/4.1.0+1/openssl/pkey.rb:394:in 'OpenSSL::PKey::RSA.new'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request.rb:64:in 'Gem::Request.configure_connection_for_https'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request/https_pool.rb:7:in 'Gem::Request::HTTPSPool#setup_connection'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request/http_pool.rb:43:in 'Gem::Request::HTTPPool#make_connection'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request/http_pool.rb:23:in 'Gem::Request::HTTPPool#checkout'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request.rb:136:in 'Gem::Request#connection_for'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request.rb:194:in 'Gem::Request#perform_request'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/request.rb:161:in 'Gem::Request#fetch'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/remote_fetcher.rb:326:in 'Gem::RemoteFetcher#request'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/remote_fetcher.rb:217:in 'Gem::RemoteFetcher#fetch_http'
/home/jaruga/var/git/ruby/rubygems/lib/rubygems/remote_fetcher.rb:261:in 'Gem::RemoteFetcher#fetch_path'
/home/jaruga/var/git/ruby/rubygems/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb:98:in 'block in TestGemRemoteFetcherLocalSSLServer#test_pqc_ssl_client_cert_auth_connection'
      95:       ":ssl_ca_cert: #{temp_ca_cert}\n" \
      96:       ":ssl_client_cert: #{temp_client_cert}\n"
      97:     ) do |fetcher|
  =>  98:       fetcher.fetch_path("https://localhost:#{ssl_server.addr[1]}/yaml")
      99:     end
     100:   end
     101:
/home/jaruga/var/git/ruby/rubygems/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb:174:in 'TestGemRemoteFetcherLocalSSLServer#with_configured_fetcher'
/home/jaruga/var/git/ruby/rubygems/test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb:94:in 'TestGemRemoteFetcherLocalSSLServer#test_pqc_ssl_client_cert_auth_connection'
```

In test/rubygems/test_gem_remote_fetcher_local_ssl_server.rb,
created new tests, test_pqc_ssl_connection and test_pqc_ssl_client_cert_auth_connection
The `start_ssl_server` has 2 modes: :non_pqc (default) and :pqc.
With the mode :pqc, `start_ssl_server` runs with the RubyGems single PQC server with
ML-KEM (X25519MLKEM768) key exchange and ML-DSA-65 certification.

Selected X25519MLKEM768 because rubygems.org supports X25519MLKEM768 for now.
Selected ML-DSA-65 because it is used and tested
https://github.com/ruby/openssl/blob/master/test/openssl/test_ssl.rb
- test_pqc_sigalg

Created `tool/create_mldsa65_certs.sh` to create ML-DSA-65 cert files,
`test/rubygems/mldsa65_*.pem`. It is inspired by `tool/create_certs.sh` to
create RSA cert files, `test/rubygems/*.pem`. Note the 65 in ML-DSA-65 is not
bit length. ML-DSA-65 is algorithm name.

Author: junaruga

lib/rubygems/request.rb

@@ -61,7 +61,7 @@ def self.configure_connection_for_https(connection, cert_files)
     if Gem.configuration.ssl_client_cert
       pem = File.read Gem.configuration.ssl_client_cert
       connection.cert = OpenSSL::X509::Certificate.new pem
-      connection.key = OpenSSL::PKey::RSA.new pem
+      connection.key = OpenSSL::PKey.read pem
     end
 
     store.set_default_paths

test/rubygems/mldsa65_ca_cert.pem

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