1/3 Use `MessageSendInstructions` instead of `PendingOnionMessage`

1/3 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 first step towards untangling that, moving from
`PendingOnionMessage` to `MessageSendInstructions` for the outbound
message queue in `CustomMessageHandler`. Better, we can also drop
the `c_bindings`-specific message queue variant, unifying the APIs.