Allow authenticating (getting token) without changing state of OcClient

[tmenier]

OcClient allows you to optionally provide an ad-hoc token on calls to any endpoint, overriding the default token stored within the OcClient instance. However, you can't really get a token in an ad-hoc way. When you call AuthenticateAsync, you're changing the default token used on the OcClient, making it hard to get different tokens using the same instance in a thread-safe way.