Add an opt-in / opt-out for Retry in ApiProvider.request

[caspervdw]

Sometimes we want to retry POST methods (which are not in the hardcoded "retryable" methods). For instance in the token retrieval in oauth2/client_credentials.py.

Also maybe: sometimes we don't want to retry methods that we are currently retrying.

I think it would be best to append the API of [Sync]ApiProvider with a retry: bool parameter.