Allow simple way to replace Request/Response or Receiver/Sender classes

[yoongkang]

Users may want to supply their own class and we might want to allow them to configure this (like Flask). This would allow them to use, e.g Starlette.

Might have challenges due to a different interface, but shouldn't be a huge problem. The protocol handler needs to be decoupled from the receiver/sender objects, so that we're not locked in to a particular class.