chore(3p/gerrit_plugins): init oauth

Add the OAuth gerrit plugin to our mini collection of Gerrit plugins.

This includes a patch to make the plugin work correctly with CAS 6.x,
which has changed the attributes into a JSON object with the attributes
nested inside, instead of a JSON list.

Change-Id: I4741f137cca9c8eb45b9ea660fb4cbf6962be9a4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2782
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

third_party/gerrit_plugins/builder.nix

@@ -7,6 +7,7 @@
     overlayPluginCmd ? ''
       cp -R "${src}" "$out/plugins/${name}"
     '',
+    postPatch ? "",
   }: ((depot.third_party.gerrit.override {
     name = "${name}.jar";
 
@@ -24,5 +25,9 @@
     installPhase = ''
       cp "bazel-bin/plugins/${name}/${name}.jar" "$out"
     '';
+    postPatch = if super ? postPatch then ''
+      ${super.postPatch}
+      ${postPatch}
+    '' else postPatch;
   }));
 }