Add source reference to BlindedPathCandidate hop

Add source reference to BlindedPathCandidate hop

Blinded paths currently contain a node id for the introduction node.
However, a compact representation will allow a directed short channel id
instead. Update BlindedPathCandidate and OneHopBlindedPathCandidate to
store a NodeId reference from either the NetworkGraph or from the user-
provided first hops.

This approach avoids looking up the introduction node id on demand,
which may not be resolvable. Thus, that would require returning an
Option from CandidateRouteHop::source and handle None accordingly.