feat(3p/nix): Add main function for grpc daemon

Implement the main function for the new, proto-based nix daemon. This
mostly replicates the behavior of the previous nix daemon - it starts a
grpc server listening on the same nix socket path and supports the same
set of options - with the exception of --stdio, which has been renamed
to --pipe and documented in the man page.

Change-Id: Ib729283c1d5d35c0e1b0a968bc1f052f5527f2d7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1356
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: kanepyork <rikingcoding@gmail.com>

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

@@ -7,7 +7,6 @@
 
 namespace nix::daemon {
 
-std::unique_ptr<nix::proto::WorkerService::Service> NewWorkerService(
-    nix::Store*);
+nix::proto::WorkerService::Service* NewWorkerService(nix::Store&);
 
 }  // namespace nix::daemon