feat(3p/nix/nix-daemon): add factory function for WorkerServiceImpl

Add a forward-declared factory function for constructing and returning a
WorkerServiceImpl, for eventual use in the main function for the nix
daemon

Change-Id: I9032d69b6ee3bc3b1f39f3d5d55f951cffad8145
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1293
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/nix-daemon/nix-daemon-proto.hh

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <memory>
+
+#include "libproto/worker.grpc.pb.h"
+#include "libstore/store-api.hh"
+
+namespace nix::daemon {
+
+std::unique_ptr<nix::proto::WorkerService::Service> NewWorkerService(
+    nix::Store*);
+
+}  // namespace nix::daemon