Add a `MessageSendInstructions::ForReply`

Add a `MessageSendInstructions::ForReply`

In order to allow onion message handlers to reply asynchronously
without introducing a circular dependency graph, the message
handlers need to be able to send replies described by
`MessageSendInstructions`. This allows them to send replies via the
normal message queuing (i.e. without making a function call to
`OnionMessenger`).

Here we enable that by adding a `MessageSendInstructions::ForReply`
variant which holds `ReplyInstruction`s.

Fixes #3178