assertive flow (GCP service accounts) RFC7521

[chenb67]

https://tools.ietf.org/html/rfc7521

https://developers.google.com/identity/protocols/oauth2/service-account#authorizingrequests

working example here - https://github.com/lepture/authlib/blob/master/authlib/integrations/httpx_client/assertion_client.py

[Colin-b]

Hi,

I won't be able to work on this before 2 weeks (at least). So if you feel like submitting a pull request in the meantime go ahead.