feat(3p/nix/nix-daemon): Implement Worker::BuildDerivation handler

Implement the proto handler on the server side for
Worker::BuildDerivation. This includes several additions to the proto
which I had missed on the first pass, including the actual proto
definition for the Derivation itself and a few sequence number
reorderings which are fine because this is all provisional and not
deployed yet.

A couple things to note

- I implemented a couple constructors for nix classes that initialize
  themselves based on their proto variants, which felt nice and didn't
  end up causing any issues.
- I've made the conversions between the enum types in nix and in proto
  explicit via switch statements rather than using a static_cast, out of
  an abundance of caution that the error would get mismatched in the
  future and we'd convert the wrong thing to the wrong thing - this is
  verbose, but exceptionally future proof.

Change-Id: Iecf6b88e76bc37e49efa05fd65d6cd0cb0deffed
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1249
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Kane York <rikingcoding@gmail.com>

third_party/nix/src/libstore/store-api.hh

@@ -8,6 +8,7 @@
 #include <unordered_map>
 #include <unordered_set>
 
+#include "libproto/worker.pb.h"
 #include "libstore/crypto.hh"
 #include "libstore/globals.hh"
 #include "libutil/config.hh"
@@ -181,6 +182,10 @@ typedef std::list<ValidPathInfo> ValidPathInfos;
 
 enum BuildMode { bmNormal, bmRepair, bmCheck };
 
+// Convert the proto version of a `nix::proto::BuildMode` to its corresponding
+// nix `BuildMode`
+std::optional<BuildMode> build_mode_from(nix::proto::BuildMode mode);
+
 struct BuildResult {
   /* Note: don't remove status codes, and only add new status codes
      at the end of the list, to prevent client/server
@@ -218,6 +223,10 @@ struct BuildResult {
   bool success() {
     return status == Built || status == Substituted || status == AlreadyValid;
   }
+
+  // Convert the status of this `BuildResult` to its corresponding
+  // `nix::proto::BuildStatus`
+  nix::proto::BuildStatus status_to_proto();
 };
 
 class Store : public std::enable_shared_from_this<Store>, public Config {