lnrpc: add new recursive proto generation script

In this commit, we add a recursive proto generation script. This avoids
having to add a new script for each upcoming sub-server.

lnrpc/signrpc/signer.pb.go

@@ -1,11 +1,11 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
-// source: signer.proto
+// source: signrpc/signer.proto
 
 /*
 Package signrpc is a generated protocol buffer package.
 
 It is generated from these files:
-	signer.proto
+	signrpc/signer.proto
 
 It has these top-level messages:
 	KeyLocator
@@ -441,10 +441,10 @@ var _Signer_serviceDesc = grpc.ServiceDesc{
 		},
 	},
 	Streams:  []grpc.StreamDesc{},
-	Metadata: "signer.proto",
+	Metadata: "signrpc/signer.proto",
 }
 
-func init() { proto.RegisterFile("signer.proto", fileDescriptor0) }
+func init() { proto.RegisterFile("signrpc/signer.proto", fileDescriptor0) }
 
 var fileDescriptor0 = []byte{
 	// 465 bytes of a gzipped FileDescriptorProto