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/gerrit-tvl/default.nix

@@ -0,0 +1,33 @@
+{ depot, pkgs, lib, ... }:
+
+let
+  classPath = lib.concatStringsSep ":" [
+    "${depot.third_party.gerrit}/share/api/extension-api_deploy.jar"
+  ];
+in
+pkgs.stdenvNoCC.mkDerivation rec {
+  name = "${pname}-${version}.jar";
+  pname = "gerrit-tvl";
+  version = "0.0.1";
+
+  src = ./.;
+
+  nativeBuildInputs = with pkgs; [
+    jdk
+  ];
+
+  buildPhase = ''
+    mkdir $NIX_BUILD_TOP/build
+
+    # Build Java components.
+    export JAVAC="javac -cp ${classPath} -d $NIX_BUILD_TOP/build --release 11"
+    $JAVAC ./HttpModule.java
+
+    # Install static files.
+    cp -R static $NIX_BUILD_TOP/build/static
+  '';
+
+  installPhase = ''
+    jar --create --file $out --manifest $src/MANIFEST.MF -C $NIX_BUILD_TOP/build .
+  '';
+}