Add GeckoView on Android

[jkbz64]

Is your feature request related to a problem? Please describe.
Android's WebView rendering model is very slow compared to rendering model that is used directly in the browser, the app opened directly in browser or opened as PWA is much faster than when it's inside WebView, it also lacks some of the Web API's. The Chromium team "is not going to change that anytime soon".

Describe the solution you'd like
Implement GeckoView as optional WebView backend on Android.

Describe alternatives you've considered
I don't think there are any other reasonable alternatives.

Additional context
It is also a very popular issue in Capacitor community and it's going to be an issue for Tauri too.