Consolidate the generation of the token result

[atn832]

Right now, getIdToken and getIdTokenResult return different pieces of information (for example the authentication and expiration dates are different). However they should be based on the same information.

Judging from the FirebaseAuth code, getIdToken and getIdTokenResult are based on the same data. So ideally [our] payload generation should be consolidated into a shared function.

• https://github.com/firebase/flutterfire/blob/dad6b4813c682e35315dda3965ea8aaf5ba030e8/packages/firebase_auth/firebase_auth_platform_interface/lib/src/method_channel/method_channel_user.dart#L42-L78
• https://github.com/firebase/flutterfire/blob/982bdfb5fbfae4a68e1af6ab62a9bd762891b217/packages/firebase_auth/firebase_auth_platform_interface/lib/src/id_token_result.dart#L16

Originally posted by @atn832 in #73 (comment)