Allow use of JWT access tokens instead of service account keys

[thequailman]

Problem description

This library requires a service account key to use. It should alternatively support JWT access tokens directly (what you exchange the service account key for with GCP). These can be issued by secret managers like Vault, and you aren't limited to 10 per service account.

Potential solutions/workarounds

Provide an environment variable for access tokens.

Additional context

[apognu]

This would also be interesting to be able to make use of other means of authenticating against a service account, for example impersonation, for more advanced authentication schemes.