Include `maybe_announced` field in `RouteHop`

Include `maybe_announced` field in `RouteHop`

When sending preflight probes, we want to exclude last hops that are
possibly announced. To this end, we here include a new field in
`RouteHop` that will be `true` when we either def. know the hop to be
announced, or, if there exist public channels between the hop's
counterparties that this hop might refer to (i.e., be an alias for).