- Currently, handle_message (or handle_custom_message) only exposes
  the generated response for an OnionMessage, lacking the necessary
  reply_path for asynchronous responses.

- This commit introduces a new flow for OnionMessage handling.

- Instead of solely taking the message as input, handle_message now accepts
  an Optional `responder`, returning a `ResponseInstruction` containing both
  the response and the reply_path.

- `ResponseInstruction` utilizes different enum variants to indicate how
  `handle_onion_message_response` should handle the response.

- This enhancement enables exposing the reply_path alongside the response
  and allows for more complex response mechanisms, such as responding with
  an added reply_path.

- The commit introduces the foundational framework (structs & enums) for this new flow.

This commit integrates the newly introduced ResponseInstruction structs
and enums into the codebase for handling OnionMessage responses.