[Firebase Auth] added access to User metadata

[LucasParsy]

allows to recover the account creation date, and the last sign in date of an user.

With them, you can determine if a user logs in for the first time.
(and then, create a welcome/ setup page, for example)

this metadata field was available in both native API, as shown in the documentation.
https://developers.google.com/android/reference/com/google/firebase/auth/FirebaseUser.html#getMetadata()

I implemented its access in the Dart library.

example usage:

  final FirebaseAuth _auth = FirebaseAuth.instance;
  FirebaseUser _currentUser = await _auth.currentUser();
  if (user.metadata.creationTimestamp ==  user.metadata.lastSignInTimestamp)
     ...

[kroikie]

@LucasParsy This change looks good to me, thanks! Could you please make and update to the change log and bump the version number

[LucasParsy]

I merged my change with the HEAD. (8178bf1)

[LucasParsy]

hello, could you merge my commit?
I updated it to work with the master branch, and I would like to use the official hosted plugin instead of my local one.
and according to the thumbs ups, some people are interested in this feature too

[kroikie]

@LucasParsy thanks for the PR!