feat: accessing granted scopes

[bshaffer]

b/253077796

Adds support for accessing granted scopes on the OAuth2 service class

Q: Does this need to be accessible by the end user?

• Yes

Q: If so, how will this be accessible by the end user?

• Exposing a method such as getGrantedScopes on the Credentials object is sufficient for this library.

Q: How is this different from the existing getScopes method?

• It wouldn't be any different for the other credentials types, only for OAuth2 and UserRefreshCredentials, because they get information back from the API

[TimurSadykov]

Sorry for the late comment, sorry if any confusion. Any reason why scopes were shared via credential, not the access token?

[bshaffer]

Hi @TimurSadykov !
If you mean why it isn't part of the class AccessToken, that's because that class is only a utility class used for validation and not used by the downstream client libraries during authentication. The credentials are the objects accessed by the downstream client libraries