Package client implements OAuth2 authentication for outbound connections from Appengine using the application services account.
GetAccessToken returns an OAuth access token representing app's service account.
If scopes is empty, uses auth.OAuthScopeEmail scope.
Implements a caching layer on top of GAE's GetAccessToken RPC. May return transient errors.
NewTokenSource makes oauth2.TokenSource implemented on top of GetAccessToken.
It is bound to the given context.