Add parse_response extension point to HTTP adapter

[wuarmin]

Hey @rmosolgo,

this PR is just a refactoring and moves the response handling of GraphQL::Client::HTTP#execute into a public parse_response(response) method, an extension point like headers and connection.

Why: Statuses other than 200 and 400 become GraphQL-style errors, so a 503 looks like any other GraphQL error. To handle HTTP statuses yourself, you currently have to copy execute. With the hook, you override just the response handling:

HTTP = GraphQL::Client::HTTP.new("https://example.com/graphql") do
  def parse_response(response)
    return super if response.is_a?(Net::HTTPOK) || response.is_a?(Net::HTTPBadRequest)

    raise MyApp::UpstreamError, "HTTP #{response.code}"
  end
end

No behavior change, unless a subclass already defines its own parse_response. Includes tests and a short section in guides/handling-errors.md.

I would be very grateful for a merge and a release. It would streamline the implementation.

Thanks
Armin