Add a geolocationPermissionsRequested event

## Use case

I'd like to be able to request geolocation runtime system permissions from the user when the web page shown in the webview plugin requests geolocation permissions. My code should then be able to tell the web site whether the permissions were given or not.

## Proposal

I'd like to have an event like `geolocationPermissionsRequested` in which I can check/request runtime permissions and return the result of that check/request. Once my (async!) code has finished and the event handler returns (the `Future` finishes), the web page should be informed of the result.

On Android, I'd basically like to implement `onGeolocationPermissionsShowPrompt` myself in Dart code. Only when my code finishes, the `GeolocationPermissions.Callback` should be called (with my result). I don't know if/how this would be done on iOS.