influxdb2-cli: install shell completion

The influx command can generate completion scripts. This PR adds a
post-intall script to install these completions for bash & zsh.

Tested the functionality for aarch64-darwin with zsh.

`influx completion --help`

> NAME:
>     completion - Generates completion scripts
>
> USAGE:
>     completion [bash|zsh|powershell]

pkgs/servers/nosql/influxdb2/cli.nix

@@ -1,6 +1,8 @@
 { buildGoModule
 , fetchFromGitHub
+, installShellFiles
 , lib
+, stdenv
 }:
 
 let
@@ -18,11 +20,19 @@ in buildGoModule {
   version = version;
   inherit src;
 
+  nativeBuildInputs = [ installShellFiles ];
+
   vendorHash = "sha256-Ov0TPoMm0qi7kkWUUni677sCP1LwkT9+n3KHcAlQkDA=";
   subPackages = [ "cmd/influx" ];
 
   ldflags = [ "-X main.commit=v${version}" "-X main.version=${version}" ];
 
+  postInstall = lib.optionalString (stdenv.buildPlatform.canExecute stdenv.hostPlatform) ''
+    installShellCompletion --cmd influx \
+      --bash <($out/bin/influx completion bash) \
+      --zsh  <($out/bin/influx completion zsh)
+  '';
+
   meta = with lib; {
     description = "CLI for managing resources in InfluxDB v2";
     license = licenses.mit;