[Enhancement] Add binding for com.google.android.libraries.ads.mobile.sdk:ads-mobile-sdk (GMA Next-Gen SDK)

[jonathanpeppers]

Google now documents the GMA Next-Gen SDK as the migration target from the current Google Mobile Ads SDK. It ships under a new artifact and groupId:

com.google.android.libraries.ads.mobile.sdk:ads-mobile-sdk:1.1.1

Maven: https://dl.google.com/dl/android/maven2/com/google/android/libraries/ads/mobile/sdk/ads-mobile-sdk/1.1.1/ads-mobile-sdk-1.1.1.pom

This is a new groupId (com.google.android.libraries.ads.mobile.sdk) — distinct from the existing com.google.android.gms:play-services-ads* artifacts — so it needs its own config.json entry and source folder rather than replacing/updating any existing binding.

Context

Came up while discussing #1428 / #1431, where play-services-ads-lite became an empty stub upstream. The Next-Gen SDK is Google's documented migration path going forward, so a .NET for Android binding would unblock consumers who want to move off the legacy GMA SDK.

Suggested NuGet ID

Something like Xamarin.Google.Android.Libraries.Ads.Mobile.Sdk (matching the convention used for other com.google.android.libraries.* bindings).

cc @tipa