* Use an interface for OIDC Client, so that we're testing the behavior
of the client, not the go-oidc package itself
* add backoff and retry when server rejects token
This commit handles:
* Passing ID Token as Bearer token
* Refreshing of tokens using refresh-tokens
* Persisting refreshed tokens
* ability to add arbitrary extra scopes via config
* this is what enables the cross-client/azp stuff