Check whether caching is correct in `req_oauth_client_credentials()` and `req_oauth_bearer_jwt()`

[mgirlich]

Similar to #178 where the caching didn't respect when the refresh token changed (e.g. in the context of supporting multiple accounts from the same client). See #179 what was done so far for the refresh token case.

[hadley]

req_oauth_client_credentials() doesn't need it because their auth is explicitly bound to the client.

req_oauth_bearer_jwt() should include the claim in the cache key.