feat: initial helm integration (thanks to @matejc)

helm2json was first implemented by matejc in
https://github.com/matejc/helm2json and incorperated in this project,
big thanks to @matejc for making this possible

helm/fetchhelm.nix

@@ -0,0 +1,48 @@
+{ stdenvNoCC, lib, kubernetes-helm, cacert }:
+
+let
+  cleanName = name: lib.replaceStrings ["/"] ["-"] name;
+
+in {
+  # name of the chart
+  chart
+
+  # chart url to fetch from custom location
+, chartUrl ? null
+
+  # version of the chart
+, version ? null
+
+# chart hash
+, sha256
+
+# whether to extract chart
+, untar ? true
+
+# use custom charts repo
+, repo ? null
+
+# pass --verify to helm chart
+, verify ? false
+
+# pass --devel to helm chart
+, devel ? false }: stdenvNoCC.mkDerivation {
+  name = "${cleanName chart}-${if version == null then "dev" else version}";
+
+  buildCommand = ''
+    export HOME="$PWD"
+    helm init --client-only >/dev/null
+    ${if repo == null then "" else "helm repo add repository ${repo}"}
+    helm fetch -d ./chart \
+      ${if untar then "--untar" else ""} \
+      ${if version == null then "" else "--version ${version}"} \
+      ${if devel then "--devel" else ""} \
+      ${if verify then "--verify" else ""} \
+      ${if chartUrl == null then (if repo == null then chart else "repository/${chart}") else chartUrl}
+    cp -r chart/*/ $out
+  '';
+  outputHashMode = "recursive";
+  outputHashAlgo = "sha256";
+  outputHash = sha256;
+  nativeBuildInputs = [ kubernetes-helm cacert ];
+}