2/3 Use `MessageSendInstructions` instead of `PendingOnionMessage`

2/3 Use `MessageSendInstructions` instead of `PendingOnionMessage`

Now that the `MessageRouter` can `create_blinded_paths` forcing
callers of the `OnionMessenger` to provide it with a reply path up
front is unnecessary complexity, doubly so in message handlers.

Here we take the next step towards untangling that, moving from
`PendingOnionMessage` to `MessageSendInstructions` for the outbound
message queue in `OffersMessageHandler`. Better, we can also drop
the `c_bindings`-specific message queue variant, unifying the APIs.

Because `ChannelManager` needs to actually control the reply path
set in individual messages, however, we have to halfway this patch,
adding a new `MessageSendInstructions` variant that allows
specifying the `reply_path` explicitly. Still, because other message
handlers are moving this way, its nice to be consistent.