feat(3p/nix): Implement FindRoots, CollectGarbage

Implement the RPC client and server handlers for the FindRoots and
CollectGarbage RPC calls

Change-Id: Ifa5d582c6a33bd1e7661ac2fc860505ef404dad0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1656
Tested-by: BuildkiteCI
Reviewed-by: kanepyork <rikingcoding@gmail.com>

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

@@ -80,8 +80,13 @@ struct GCOptions {
 
   /* Stop after at least `maxFreed' bytes have been freed. */
   unsigned long long maxFreed{std::numeric_limits<unsigned long long>::max()};
+
+  [[nodiscard]] const proto::GCAction ActionToProto() const;
 };
 
+std::optional<GCOptions::GCAction> GCActionFromProto(
+    nix::proto::GCAction gc_action);
+
 struct GCResults {
   /* Depending on the action, the GC roots, or the paths that would
      be or have been deleted. */