feat: send quota project id in x-goog-user-project for OAuth2 credentials (#412)

* feat: send quota project id in x-goog-user-project header for OAuth2 credentials (#400)

When the 3LO credentials are used, the quota project ("quota_project_id") is sent on every outgoing request in the x-goog-user-project HTTP header/grpc metadata. The quota project is used for billing and quota purposes.

* feat: add `__setstate__` and `__getstate__` to `oauth2.credentials` class

5 files changed