Implement Xamarin.Essentials.WebAuthenticator

[TheCodeTraveler]

WebAuthenticator utilizes ASWebAuthenticationSession on iOS which allows for more flexibility in setting session ids and cookies.

https://docs.microsoft.com/xamarin/essentials/web-authenticator

[TheCodeTraveler]

Xamarin.Forms.WebAuthenticator is hamstrung by the iOS / Android APIs which don't allow the customization of Browser.

Putting this on hold for now.

[TheCodeTraveler]

Work being done in this branch: https://github.com/brminnick/GitTrends/tree/Xamarin.Essentials.WebAuthenticator