Provide an interface to attach child processes to a stream

[essen]

With WebTransport or HTTP/2+ Websocket we may have multiple WebTransport or Websocket sessions on a single connection, so we need a way to attach processes per stream rather than per connection (which is accomplished by starting a supervisor before the connection process). We already can have stream children, but there's no convenient way to attach these children from a request process in Cowboy. cowboy_stream_h needs to return a spawn message in info when the request process tells it to.