brew: setup shell completion

[jkroepke]

This PR add the automatic setup of shell completion for StackIT CLI.

Docs: https://rubydoc.brew.sh/Formula#generate_completions_from_executable-instance_method

Ref for kubectl: https://github.com/Homebrew/homebrew-core/blob/33a3f1b400026561ff01cd8f7647a0503a06cbf7/Formula/k/kubernetes-cli.rb#L39C5-L39C41

Tested locally:

jok@jok-air ~ % curl https://raw.githubusercontent.com/jkroepke/homebrew-tap/refs/heads/patch-1/Formula/stackit.rb -O
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  2007  100  2007    0     0   3778      0 --:--:-- --:--:-- --:--:--  3779
jok@jok-air ~ % brew install stackit.rb
Error: Failed to load cask: stackit.rb
Cask 'stackit' is unreadable: wrong constant name #<Class:0x00000001257233e8>
Warning: Treating stackit.rb as a formula.
==> Fetching stackit
==> Downloading https://github.com/stackitcloud/stackit-cli/releases/download/v0.26.0/stackit-cli_0.26.0_darwin_arm64.tar.gz
==> Downloading from https://objects.githubusercontent.com/github-production-release-asset-2e65be/699237555/60f3124a-f258-4a4b-a1d6-9e17eaaf03db?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=releaseas
##################################################################################################################################################################################################### 100.0%
==> Caveats
zsh completions have been installed to:
  /opt/homebrew/share/zsh/site-functions
==> Summary
🍺  /opt/homebrew/Cellar/stackit/0.26.0: 9 files, 41.3MB, built in 3 seconds
==> Running `brew cleanup stackit`...
Disable this behaviour by setting HOMEBREW_NO_INSTALL_CLEANUP.
Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).

[jkroepke]

@marceljk Could you take a look here as well?

[marceljk]

Hi @jkroepke, thanks for your contribution.

homebrew-tap/Formula/stackit.rb

Line 4 in f887399

# This file was generated by GoReleaser. DO NOT EDIT.

It looks like the file is generated and with the next version of the CLI this will be overwritten. I'll check this

[marceljk]

Hi @jkroepke,

the shell completion is now added within the goreleaser and with the next release, this will be added to the homebrew tap. (see stackitcloud/stackit-cli#692)

Thanks for your contribution 😃