feat: implement base classes for credentials and request sessions

[ohmayr]

This PR implements base classes for credentials and request sessions to capture the common logic between synchronous and asynchronous code.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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[vchudnov-g]

LGTM, but get sign-off from auth team.

Quick observation: When refactoring file contents into a new file but still keeping the old file, it is helpful to reviewers if you can make the new file share the old file's previous history, so reviewers can see the diff's between the old file and the new as well as between the the old file's previous version and new version. Instructions are here: https://stackoverflow.com/a/44036771/23828460 (also see https://www.mankier.com/1/git-cp#Synopsis)

No need to do it now after the fact, but keep it in mind for future refactorings.

[vchudnov-g]

I realize this was in the original file, but it's weird having a string floating as a comment rather than a proper Python comment.

[clundin25]

My thoughts are similar here re:codesharing. Since the async code is a "colored" function I think it's safe to assume that the sync code and async code should never be in the same callstack.

[vchudnov-g]

In terms of the async semantics, yes that is a valid consideration. But there are still common parts of the logic are re-used between sync and async and those would make sense to consider factoring out. (I think in the rant you linked to, that corresponds to the "blue" functions.)

[parthea]

nox > flake8 --import-order-style=google --application-import-names=google,tests,system_tests google tests tests_async
google/auth/credentials.py:26:1: I100 Import statements are in the wrong order. 'from google.auth._credentials_base import _BaseCredentials' should be before 'from google.auth._refresh_worker import RefreshThreadManager'
google/auth/transport/requests.py:41:1: I100 Import statements are in the wrong order. 'import google.auth.transport._mtls_helper' should be before 'from google.auth.transport._requests_base import _BaseAuthorizedSession'
nox > Command flake8 --import-order-style=google --application-import-names=google,tests,system_tests google tests tests_async failed with exit code 1
nox > Session lint failed.