Support opening auth url on FreeBSD

[gogolok]

Description

stackit-cli can be used with key flow, but cannot be used right now using stackit auth login.
You will receive an 'unsupported platform' error when trying to open the browser URL to authenticate.

FreeBSD can be used similar to Linux, so I just added it next to Linux in openBrowser. I think the WSL_DISTRO_NAME lookup is not relevant for FreeBSD, but also doesn't hurt and didn't want to create a separate switch case. But let me know if you prefer a separate switch case.

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)