[Firebase Auth] - MacOS support for SigninWithProvider

[Mooyeee]

This PR adds support for the method signInWithProvider (from the flutter SDK) for macOS, allowing us to login with SSO on macOS platform. Tried with Microsoft AD SSO. The proposed solution mimics the iOS one but implemented of course with AppKit.

Linked Issues: https://github.com/firebase/firebase-ios-sdk/issues/14990

Let me know if something looks odd :)

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[Mooyeee]

I've noticed some issues and fixed them :) The issues:

• The user closing flow wasn't correctly notified to flutter, overriding close instead of performClose fixed it
• The first frame of the NSWindow was white until a resize; I was mistakenly creating two spinners and two webviews, creating them once fixed the issue

Also formatted the code

[Vincz]

Any update regarding the merging of this amazing PR?