[FEATURE] Add autofill support

[jaroslawdabrowski]

Is your feature request related to a problem? Please describe.
In login forms it is crucial to have autofill of username and password. If you logout, you don't want to reinsert username and password, but want system to propose the securely stored login data.

Nice to have: propose new password. E.g. iPhone suggests secure password.

Describe the solution you'd like
I saw that there is already an option for that in flutter. https://stackoverflow.com/questions/55633695/flutter-password-autofill. AutoFillGroup - https://api.flutter.dev/flutter/widgets/AutofillGroup-class.html

Additional context
Please let me know if you are short in resources, perhaps I will find couple of hours to implement it and send you a pull request. It is rather critical for us to go live with your flutter_login.

BTW. flutter_login is great, it saved us many dev hours.