Add STACKIT

[robinschneider]

Add new provider STACKIT for testing.

Add CI automation for vendor testing of provider STACKIT.

How to use

Run in CI or locally with all required environment variables.

Testing done

[Describe the testing you have done before submitting this PR. Please include both the commands you issued as well as the output you got.]

• [ ] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.