[firebase_auth] Confirm password reset

[lanmonster]

Description

This PR gives the ability to confirm password reset in order to control the entire "forgot password" flow from within an app.

Related Issues

#1739

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[bgetsug]

@lanmonster I've got an implementation of confirmPasswordReset for firebase_auth_web (which prompted my submission of #1780). There's really not much to it, if you'd like to add it to this PR, otherwise I'm happy to open a separate one:

packages/firebase_auth/firebase_auth_web/lib/firebase_auth_web.dart:

  Future<void> confirmPasswordReset(
    String app,
    String code,
    String newPassword,
  ) {
    final firebase.Auth auth = _getAuth(app);
    return auth.confirmPasswordReset(code, newPassword);
  }

[lanmonster]

@bgetsug happy to include that in my PR, but I think it will have to wait until #1780 lands.

[collinjackson]

confirmPasswordReset in #1780 has landed, can you please update this PR to use it?

[lanmonster]

@collinjackson I updated the PR