Add an example/integration test that injects a Credential

[ge0ffrey]

There are many other Google APIs than those already supported out of the box, such as the youtube API to get the number of viewers on a video, or Google Sheets to drop that data in a spreadsheet.

Supporting all those is not realistic at the moment, but all those APIs do need the Google Credentials to log in. By adding an example how to inject those, it become easier to use those extra APIs manually.

[loicmathieu]

Hi,
Yes, it was on my TODO list to explain how to use this library as a foundation for supporting another Google library.
The idea was to use as an example for eg this PR and explain how to do it with Auth management, accessing the projectId property and dealing with gRPC support if needed.