invoicesrpc: create sub server

Sub server implementation is still empty. This is a preparatory
step for adding invoice functionality.

lnrpc/invoicesrpc/config_default.go

@@ -0,0 +1,6 @@
+// +build !invoicesrpc
+
+package invoicesrpc
+
+// Config is empty for non-invoicesrpc builds.
+type Config struct{}