Add support for local web server auth flow

[chanseokoh]

Something like LocalServerReceiver in google-oauth-client.

[chingor13]

This is outside the scope of this repository. The implementation(s) in google-oauth-client integrate with several Java frameworks and will work with any OAuth2 implementation (not just Google's).

Once you receive your credentials from the OAuth flow, you can instantiate a Credentials implementation here:

// import com.google.auth.Credentials;
// import com.google.auth.oauth2.AccessToken;
// import com.google.auth.oauth2.OAuth2Credentials;

AccessToken accessToken = new AccessToken(
    "your-access-token-from-auth-flow", expirationFromAuthFlow);
Credentials credentials = OAuth2Credentials.create(accessToken);