kaf: add shell completions

pkgs/by-name/ka/kaf/package.nix

@@ -2,6 +2,8 @@
   lib,
   buildGoModule,
   fetchFromGitHub,
+  installShellFiles,
+  stdenv,
 }:
 
 buildGoModule rec {
@@ -17,9 +19,18 @@ buildGoModule rec {
 
   vendorHash = "sha256-1QcQeeYQFsStK27NVdyCAb1Y40lyifBf0dlSgzocG3Y=";
 
+  nativeBuildInputs = [ installShellFiles ];
+
   # Many tests require a running Kafka instance
   doCheck = false;
 
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd kaf \
+      --bash <($out/bin/kaf completion bash) \
+      --zsh <($out/bin/kaf completion zsh) \
+      --fish <($out/bin/kaf completion fish)
+  '';
+
   meta = with lib; {
     description = "Modern CLI for Apache Kafka, written in Go";
     mainProgram = "kaf";