routerrpc: add HasNode backend function for LSP heuristic

This commit adds the HasNode function to the RouterBackend struct,
which checks if a node exists in the graph (i.e., has public channels).
This function is needed by the LSP detection heuristic to determine
if a node is publicly reachable.

The function is wired up in rpcserver.go to query the graph database.

rpcserver.go

@@ -695,6 +695,8 @@ func newRPCServer(cfg *Config, interceptorChain *rpcperms.InterceptorChain,
 // addDeps populates all dependencies needed by the RPC server, and any
 // of the sub-servers that it maintains. When this is done, the RPC server can
 // be started, and start accepting RPC calls.
+//
+//nolint:funlen
 func (r *rpcServer) addDeps(ctx context.Context, s *server,
 	macService *macaroons.Service,
 	subServerCgs *subRPCServerConfigs, atpl *autopilot.Manager,
@@ -744,6 +746,11 @@ func (r *rpcServer) addDeps(ctx context.Context, s *server,
 
 			return info.NodeKey1Bytes, info.NodeKey2Bytes, nil
 		},
+		HasNode: func(nodePub route.Vertex) (bool, error) {
+			_, exists, err := graph.HasNode(ctx, nodePub)
+
+			return exists, err
+		},
 		FindRoute:              s.chanRouter.FindRoute,
 		MissionControl:         s.defaultMC,
 		ActiveNetParams:        r.cfg.ActiveNetParams.Params,