Introduce ResponseInstructions for OnionMessage Handling

[shaavan]

resolves #2882

Introduce ResponseInstructions for OnionMessage Handling

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