allow setting the faraday adapter to use

The application I work on uses httpclient and relies on its persistent connection feature and request hedging to keep p99 google storage uploads to a minimum. v1 changed to faraday, which is understandable, but the default faraday adapter uses net-http, which does not do persistent requests, and would severely limit our ability to seamlessly upgrade.

This patch introduces a new client option, :adapter, which allows setting a default adapter (faraday has several) which supports persistent connections via:

Google::Apis::ClientOptions.default.adapter = :httpx # or
:net_http_persistent

Start at the Google::Apis::ClientOptions.default entry point and trace how the Faraday client selects its adapter. Check the existing client configuration and adapter-related tests, then verify that setting :adapter to :httpx or :net_http_persistent changes the default adapter without breaking the existing behavior.

Written by the indexing model from the issue text.