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.

lnrpc/routerrpc/router_backend.go

@@ -64,6 +64,11 @@ type RouterBackend struct {
 	FetchChannelEndpoints func(chanID uint64) (route.Vertex,
 		route.Vertex, error)
 
+	// HasNode returns true if the node exists in the graph (i.e., has
+	// public channels), false otherwise. This means the node is a public
+	// node and should be reachable.
+	HasNode func(nodePub route.Vertex) (bool, error)
+
 	// FindRoute is a closure that abstracts away how we locate/query for
 	// routes.
 	FindRoute func(*routing.RouteRequest) (*route.Route, float64, error)