Move two methods from GIDAuthentication API to GIDGoogleUser API

[Alex-4-Git]

1. Add two methods in GIDGoogleUser public API.

• @property(nonatomic, readonly)(id<GTMFetcherAuthorizationProtocol>) fetcherAuthorizer;
• (void)doWithFreshTokens:(void (^)(GIDGoogleUser *_Nullable user, NSError *_Nullable error))completion;

2. Remove GIDAuthentication.h from GSI public API.
3. Keep GIDAuthentication as an internal class.

[petea]

Discussed offline and decided to resume this PR once we've documented the implementation approach for the larger GIDGoogleUser refactoring.