Implement pre-emptive credential refresh #15200

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Is your feature request related to a problem? Please describe.
In the current implementation OAuth credentials get refreshed inline when auth headers are requested by gRPC. This causes latency spikes that are hard to debug.

Describe the solution you'd like
Implement a stale state for OAuth tokens where the existing token is returned, and a refresh is triggered in the background. Prior art: googleapis/google-auth-library-java#646

Describe alternatives you've considered
Other possibilities are:

• Have an always running background thread to always refresh the token. This will have lifetime issues since credentials dont currently have a way to close them/stop the thread