An interface implemented by objects that can fetch auth tokens.
|public abstract fetchAuthToken(?callable $httpHandler = NULL)|
Fetches the auth tokens based on the current state.
|public abstract getCacheKey()|
Obtains a key that can used to cache the results of #fetchAuthToken.
If the value is empty, the auth token is not cached.
|public abstract getLastReceivedToken()|
Returns an associative array with the token and expiration time.