Add support for Microsoft auth

[eggzotic]

It would be a great addition to this package to add Microsoft sign-in alongside the already very nice Google, Github, Twitter & Facebook. Especially for Flutter native Windows apps, this would close a gap.

[Mooyeee]

Unfortunately, I think that won't be possible until firebase will have the possibility to securely identify microsoft tokens...
I tried myself both with my own implementation and with this library by creating a custom arg and in both cases, while I successfully login to AD, firebase won't accept the credential since it didn't handle the OAuth flow entirely.

Here the doc that explains it: https://firebase.google.com/docs/auth/ios/microsoft-oauth#expandable-2