feat(tvix): Implement all remaining RPC calls

Implement all remaining RPC calls on the RpcSstore client, remove a few
stub methods we had added that weren't actually present in the old
RemoteStore implementation, and add one more RPC call for getBuildLog
that is present in the store API, but that we hadn't added as a
stub *or* to the proto.

Fixes: #29
Change-Id: Id827f51a393ece4bc7bbecaf38aee9eb4b329770
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1692
Reviewed-by: kanepyork <rikingcoding@gmail.com>
Tested-by: BuildkiteCI

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

@@ -235,6 +235,9 @@ struct BuildResult {
   // Convert the status of this `BuildResult` to its corresponding
   // `nix::proto::BuildStatus`
   nix::proto::BuildStatus status_to_proto();
+
+  static std::optional<BuildResult> FromProto(
+      const nix::proto::BuildDerivationResponse& resp);
 };
 
 class Store : public std::enable_shared_from_this<Store>, public Config {