feat(goreleaser): add completion to homebrew installation

[marceljk]

Description

relates to stackitcloud/homebrew-tap#3

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[rubenhoenle]

Can this be tested somehow?

[jkroepke]

@rubenhoenle I tested the generate_completions_from_executable as part of

stackitcloud/homebrew-tap#3 - I added the commands to test it.

[marceljk]

To get the generated Formula, you need to run $ goreleaser release --skip=publish,validate --clean. But you need to remove some steps temporary from the .goreleaser.yaml, because they depends on some credentials or try to publish something. I described it more in our internal channel.
And with the generated Formula, the test from @jkroepke can be executed