cln: Migration from plugin to gRPC daemon

[YusukeShimizu]

I propose migrating the communication between cln and peerswap, which currently uses Unix Domain Socket as a core lightning plugin, to gRPC. This migration will allow users to operate peerswap by starting the daemon and connecting via gRPC, regardless of whether they are using cln or lnd as their backend.

Benefits

• Improved Usability:
  • Users can operate peerswap with the same interface, whether they are using cln or lnd as their backend.
• Simplified System:
  • Peerswap can be operated simply by starting the daemon, simplifying the system architecture.
• Enhanced Maintainability:
  • Adopting gRPC makes it easier to follow changes in the cln interface and detect modifications.

Challenges

• Loss of Backward Compatibility:
  • Since the current cln likely does not consider backward compatibility with gRPC, peerswap will also lose its backward compatibility for cln.
• Changes in Setup Process:
  • The setup process for peerswap will need to be modified to accommodate gRPC.
  • All cln users will be required to redo their setup.
• Large Code Changes:
  • Relatively large-scale changes across the entire codebase will be necessary.