Log message:
Update py-gcs-oauth2-boto-plugin to 2.1.

- Python 3 support
- mox -> mock
- Cache RAPT to prevent frequent reauth challenges.

Part n of PR pkg/52941.

Log message:
Add a package for py-gcs-oauth2-boto-plugin.

gcs-oauth2-boto-plugin is a Python application whose purpose is to behave as an
auth plugin for the boto auth plugin framework for use with OAuth 2.0
credentials for the Google Cloud Platform. This plugin is compatible with both
user accounts and service accounts, and its functionality is essentially a
wrapper around oauth2client with the addition of automatically caching tokens
for the machine in a thread- and process-safe fashion.

Part of PR pkg/52941.