feat: add support for workforce pool credentials

[bojeil-google]

Workforce pools (external account credentials for non-Google users) are
organization-level resources which means that issued workforce pool tokens
will not have any client project ID on token exchange as currently designed.

"To use a Google API, the client must identify the application to the server.
If the API requires authentication, the client must also identify the principal
running the application."

The application here is the client project. The token will identify the user
principal but not the application. This will result in APIs rejecting requests
authenticated with these tokens.

Note that passing a x-goog-user-project override header on API request is
still not sufficient. The token is still expected to have a client project.

As a result, we have extended the spec to support an additional
workforce_pool_user_project for these credentials (workforce pools) which will
be passed when exchanging an external token for a Google Access token. After the
exchange, the issued access token will use the supplied project as the client
project. The underlying principal must still have serviceusage.services.use
IAM permission to use the project for billing/quota.

This field is not needed for flows with basic client authentication (e.g. client
ID is supplied). The client ID is sufficient to determine the client project and
any additionally supplied workforce_pool_user_project value will be ignored.

Note that this feature is not usable yet publicly.

The additional field has been added to the abstract external account credentials
google.auth.external_account.Credentials and the subclass
google.auth.identity_pool.Credentials.

[bojeil-google]

@busunkim96 and @arithmetic1728, I am running into the same recurring issue in the system tests: {'error': 'invalid_grant', 'error_description': 'Token has been expired or revoked.'}