Adds protocols to model credential storage

[mdmathias]

This change also removes saving, reading, and removing operations from the API for GTMAppAuthFetcherAuthorization and GTMOAuth2KeychainCompatibility.

[petea]

The direction here feels good.

Relatedly, here's a straw man (re)naming proposal for discussion:

GTMAppAuthFetcherAuthorization -> GTMAuthState <GTMFetcherAuthorizationProtocol, NSSecureCoding> GTMAuthStateStore GTMKeychainStore <GTMAuthStateStore> GTMOAuth2KeychainCompatibility -> GTMOAuth2CompatibilityStore <GTMAuthStateStore>

// Creates a GTMAuthState from an OIDAuthState.
GTMAuthState *authState = [[GTMAuthState alloc] initWithAuthState:appAuthState];
self.authState = authState;

// Creates a GTMSessionFetcherService with the GTMAuthState as authorizer.
GTMSessionFetcherService *fetcherService = [[GTMSessionFetcherService alloc] init];
fetcherService.authorizer = self.authState;

[mdmathias]

I like the name changes. PR updated.

[mdmathias]

The @objc name for AuthState is currently GTMAppAuthFetcherAuthorization in this change. We should use a different name for ObjC and make sure that the KeychainStore does the right migration.

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