Replace GTMFetcherAuthorizationProtocol with GTMSessionFetcherAuthorizer

[Alex-4-Git]

Context:
Deprecating GTMFetcherAuthorizationProtocol in favor of new GTMSessionFetcherAuthorizer

Solution:
We should replace the protocol type of fetcherAuthorizer in GIDGoogleUser API.

[mdmathias]

Surfaced during discussion on #230.