Make tonic-web layer generic over request/response body

## Feature Request

Right now the tonic_web layer requires that the upstream service returns a `http::Response` and it only accepts `http::Request`. It could be made generic over the request/response body so it could be used in more contexts—for example you could implement the tonic-web layer as a reverse proxy.

### Motivation

I want to [proxy a Python gRPC service with tonic-web](https://github.com/hyperium/tonic/discussions/1357). The lack of flexibility means I can't just stick tonic-web in front of a `hyper::Client` to proxy the upstream.