Compact blinded path handling

[jkczyz]

Blinded paths may be specified using a more compact representation, replacing pubkeys with short channel ids as described in #2921. This PR implements this in the following manner:

• Update BlindedPath and message::ForwardTlvs to support the compact representation
• Add a NodeIdLookUp trait used in OnionMessenger to handle forwarding
• Implement NodeIdLookUp for ChannelManager
• To resolve the IntroductionNode:
  • For messaging, use the NetworkGraph in DefaultMessageRouter
  • For payments, use either the NetworkGraph or first hops in DefaultRouter

Does not yet include any tests as the creation methods need to be updated still.