Return GraphQL response as GdsApi::Response

This is instead of `Hash`.

In Government Frontend, we need some other information from the request
response (e.g. the cache control headers) which are not included when
the response is converted to a hash.

This is not a breaking change as the new object type accepts the same
methods as the hash.

lib/gds_api/publishing_api.rb

@@ -564,7 +564,7 @@ def get_schema(schema_name)
   #
   # @return [Hash] A response with the result of the GraphQL query.
   def graphql_query(query)
-    post_json("#{endpoint}/graphql", query:).to_hash
+    post_json("#{endpoint}/graphql", query:)
   end
 
 private