feat: add a way for users to specify custom headers.

For certain scenarios, custom headers are required to be sent to the backend along with the usual headers such as `authorization`, `x-goog-user-project`. Please expose a way for users to specify these custom headers.

This issue [has a PR with a lot of work and some suggestions](https://github.com/googleapis/google-auth-library-python/pull/1811) for how to take the PR across the finish line.

The [PR](https://github.com/googleapis/google-auth-library-python/pull/1811) was **temporarily** closed to enable migration of the original repo to the `google-cloud-python` mono-repo. Please refer to the PR for suggested code changes and for next steps.