feat: send quota project id in x-goog-user-project for OAuth2 credentials

[busunkim96]

When the 3LO credentials are used, the quota project ("quota_project_id") should be sent on every outgoing request in the x-goog-user-project HTTP header/grpc metadata.

[broady]

LGTM, PTAL @shinfan / @CodingFanSteve

[shinfan]

LGTM. Thanks.

Just wanna mention we will also need this quota project as a client-level option.

@wora FYI

[busunkim96]

@shinfan @broady Thank you for the fast reviews! Is sometime late in Q1 an acceptable timeframe for adding this as a client-level option? I'd prefer to be done with the transition to the Python microgenerator to avoid doing the work twice. 😄

[shinfan]

late Q1 sounds good to me :) Thank you!

[srulkapara]

@busunkim96 just a quick note that this code is not fully backward-compatible (not talking about constrcutors - i mean in terms of already-pickled creds-token-files, who are missing the new member - who has no default on __setstate__).
Some of Google's best-practices/demo-quickstarts include pickling these tokens to files, so they will be broken after this merge (while not changing the minor module-version). maybe we should cover those cases as well..
(happened to me, thought maybe you should know)

[busunkim96]

@srulkapara Thank you for the heads up! I'll continue to track this in #405