Allow loading schema from SDL definition file as well as JSON

If passing a path to a file with a name that ends in ".graphql" or
".graphqls" (matching the filenames used in
[graphql-ruby](https://github.com/rmosolgo/graphql-ruby/blob/master/lib/graphql/schema.rb#L116)),
attempt to load the schema from that file assuming it's a schema
definition rather than a JSON schema.

Author: Tabby

lib/graphql/client.rb

@@ -58,6 +58,8 @@ def self.load_schema(schema)
           GraphQL::Schema.from_definition(schema)
         elsif schema =~ /\A\s*{/
           load_schema(JSON.parse(schema, freeze: true))
+        elsif schema.start_with?("schema")
+          GraphQL::Schema.from_definition(schema)
         end
       else
         if schema.respond_to?(:execute)

test/test_client_schema.rb

@@ -48,6 +48,16 @@ def test_load_schema_from_definition_string
     assert_equal "AwesomeQuery", schema.query.graphql_name
   end
 
+  def test_load_schema_from_definition_file
+    definition = Schema.to_definition
+    Tempfile.create(["schema", ".graphql"]) do |file|
+      file.write(definition)
+      file.close
+      schema = GraphQL::Client.load_schema(file.path)
+      assert_equal "AwesomeQuery", schema.query.graphql_name
+    end
+  end
+
   def test_load_schema_ignores_missing_path
     refute GraphQL::Client.load_schema("#{__dir__}/missing-schema.json")
   end