ListenerRegistration should implement IDisposable #746

Please fill in the following fields:

Unity editor version: All
Firebase Unity SDK version: At least 6.15.2 and lower
Source you installed the SDK (.unitypackage or Unity Package Manager): UPM
Firebase plugins in use (Auth, Database, etc.): FireStore

Please describe the issue here:

Some 3rd party made to be general listener (honestly https://github.com/neuecc/UniRx)

I want to use FireStore Listener API with it but then ListenerRegistration need to implement IDisposable for compatible with it

Could you please add it like this

	public sealed class ListenerRegistration : IDisposable
	{
		public Task ListenerTask { get; }

		public void Stop();
		public void Dispose() => listener.Stop();
	}

So I could use UniRX like so

	Observable.Create<QuerySnapshot>((observer) => {
		return colReference.Listen((snap) => observer.OnNext(snap));
	});