Feature Request: Add support for HTTP/2

[TommyN]

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 property, but as far as I can gather this requires changes in the Google.Apis.Core and requires >= .NET 5

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[ewfian]

Related: https://github.com/firebase/firebase-admin-dotnet/issues/349#issuecomment-1647384948

[JensSchadron]

@lahirumaramba Any news on this? Because of the deprecation of the FCM batch API endpoint (in 15 days from now) and the HTTP V1 API relying mostly on HTTP/2 multiplexing to achieve greater throughput, this seems like a pretty important thing to look into.