feat(functions): Initial Functions SDK

[cedweber]

Description

Hi team,

The STACKIT Functions services SDK has not been generated yet.
With this PR we would like to introduce the service and the services wait handler to the STACKIT SDK.

relates to #11374

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• VERSION file(s) of the service(s) were adjusted
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• 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)