Feature Request: Add support for HTTP/2

Modifying the `FirebaseMessaingClient.SendAsync` and adding new Version(2, 0) should allow HTTP/2 to be used.
```
...
var request = new HttpRequestMessage()
{
Version = new Version(2, 0),
Method = HttpMethod.Post,
RequestUri = new Uri(this.sendUrl),
Content = NewtonsoftJsonSerializer.Instance.CreateJsonHttpContent(body),
};
```

Users of this library would probably also want to configure the [EnableMultipleHttp2Connections](https://docs.microsoft.com/en-us/dotnet/api/system.net.http.socketshttphandler.enablemultiplehttp2connections?view=net-6.0) property, but as far as I can gather this requires changes in the Google.Apis.Core and requires >= .NET 5