Initial release of the STACKIT Edge-Cloud service CLI (BETA)

[RobertRuf]

Description

This is the initial release of the STACKIT Edge-Cloud service CLI including some general contributions to the STACKIT CLI such as custom errors and new testing utils. Relates to #1222

A internal review of the Edge-Cloud contribution to the STACKIT CLI has been done in stackit-cli-internal/pullrequest/874065.

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)