Still need to check auth_time claim is before exp claim

Abstract GoogleKeyProvider to a trait that is implemented by GoogleSignKeyProvider and FirebaseAuthenticationKeyProvider. Abstract RequiredClaims and IdPayload to associated types of the Validator trait which is implemented by GoogleSigninValidator and FirebaseValidator. Still need to expose the new API, include a check for auth_time claim for FirebaseValidator, expand FirebaseIdPayload, test FirebaseClient using custom key provider and refactor.

std::sync::Mutex::lock blocks the thread so will cause deadlocks when run inside a tokio runtime. using tokio::sync::Mutex in a TokioClient fixes this deadlock as tested in test_deadlock_prevention. It seems as though each async runtime will have to be explicitly supported

Client::firebase_builder, Client::google_signin_builder, Client::new_firebase and Client::new_google_signin to construct any kind of client from Client

No need to have to explicitly import Claims trait if method is inherent

A lot of information was being lost when converting 3rd party library errors to this crate's Error type. Error::InvalidToken is particularly unhelpful for users of this library because it represented so many different underlying errors. Error has been restructured so that it contains all underlying error information. Replaced unsafe_ignore_expiration method with unsafe_mock_timestamp method that it can be used to test firebase token validation in a deterministic way. Instead of claims_are_valid method for Validator, use validate_claims method that returns the error corresponding to a failing validation condition of the claim. This allows specific error information to propogate to the library user.

Didn't include firebase claim, couldn't find documentation explaining all possible values it could be

Also add FirebaseIdPayload and FirebaseRequiredClaims to the public API

Instead of using get_payload, using move_payload prevents unnecessary cloning of the payload