kubernetes-helm: support plugins

also introduce helm-s3, helm-diff, helm-secrets plugin.

You can create a wrapped helm with these plugins via:

  myHelm = final.wrapHelm final.kubernetes-helm-unwrapped {
    plugins = with final.kubernetes-helmPlugins; [ helm-s3 helm-secrets helm-diff ];
  };

Running `helm plugin list` will show you these are available.

pkgs/applications/networking/cluster/helm/wrapper.nix

@@ -0,0 +1,48 @@
+{ stdenv, symlinkJoin, lib, makeWrapper
+, writeText
+}:
+
+helm:
+
+let
+  wrapper = {
+    plugins ? [],
+    extraMakeWrapperArgs ? ""
+  }:
+  let
+
+  initialMakeWrapperArgs = [
+      "${helm}/bin/helm" "${placeholder "out"}/bin/helm"
+      "--argv0" "$0" "--set" "HELM_PLUGINS" "${pluginsDir}"
+  ];
+
+  pluginsDir = symlinkJoin {
+    name = "helm-plugins";
+    paths = plugins;
+  };
+in
+  symlinkJoin {
+    name = "helm-${lib.getVersion helm}";
+
+    # Remove the symlinks created by symlinkJoin which we need to perform
+    # extra actions upon
+    postBuild = ''
+      rm $out/bin/helm
+      makeWrapper ${lib.escapeShellArgs initialMakeWrapperArgs}  ${extraMakeWrapperArgs}
+    '';
+    paths = [ helm pluginsDir ];
+
+    preferLocalBuild = true;
+
+    nativeBuildInputs = [ makeWrapper ];
+    passthru = { unwrapped = helm; };
+
+    meta = helm.meta // {
+      # To prevent builds on hydra
+      hydraPlatforms = [];
+      # prefer wrapper over the package
+      priority = (helm.meta.priority or 0) - 1;
+    };
+  };
+in
+  lib.makeOverridable wrapper