Suggestion: Support for OpenID Connect

[jollytoad]

I raise this as it's been discussed elsewhere, and there is currently no explicit issue for it in this project.

There is a limitation in oauth2_client atm that prevents this: it doesn't expose the id_token, and there is no official way to
obtain that token from kv_oauth either. This can be worked around with a patch to oauth2_client (cmd-johnson/deno-oauth2-client#32) and use of the internal getTokens function.

Although experimental work is underway in oauth2_client to fully support the OIDC flow: https://github.com/cmd-johnson/deno-oauth2-client/tree/feature/oidc