feat(ops/gerrit-tvl): init TVL Gerrit plugin

This is just going to be a grab bag of things which do TVL-specific
things to Gerrit, whether that be exposing new Prolog predicates or, as
I intend to do as the first thing, expose Buildkite builds as checks.

Change-Id: Iaeab987a1fdbd078b85e274691c986489903bf3a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2872
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

ops/nixos/monorepo-gerrit.nix

@@ -18,9 +18,9 @@ in {
     ];
 
     plugins = with depot.third_party.gerrit_plugins; [
-      checks
       owners
       oauth
+      depot.ops.gerrit-tvl
     ];
 
     package = depot.third_party.gerrit;