How to obtain the default HttpHandler in order to only influence keepalive settings

Hello,

I want to enable HTTP/2 keep alive for a gRPC client. I believe that I should supply the relevant settings via the `GrpcChannelOptions.HttpHandler` property.

My question is, how should I construct a default `HttpMessageHandler`, which normally happens [here](https://github.com/grpc/grpc-dotnet/blob/7154cb470d2dd26958580881d0ba9eb634f6f49e/src/Grpc.Net.Client/GrpcChannel.cs#L384).

I only want to influence the keepalive settings, the rest should be the gRPC defaults which are used internally.

Is there something I am missing here or should those default be exposed somehow?