invoicesrpc: create sub server

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

lnrpc/invoicesrpc/config_active.go

@@ -0,0 +1,16 @@
+// +build invoicesrpc
+
+package invoicesrpc
+
+import (
+	"github.com/lightningnetwork/lnd/invoices"
+)
+
+// Config is the primary configuration struct for the invoices RPC server. It
+// contains all the items required for the rpc server to carry out its
+// duties. The fields with struct tags are meant to be parsed as normal
+// configuration options, while if able to be populated, the latter fields MUST
+// also be specified.
+type Config struct {
+	InvoiceRegistry *invoices.InvoiceRegistry
+}