refactor(cli): add ConvertStringMapToInterfaceMap util function

[Benjosh95]

Description

Codebase contained multiple implementations of converting map[string]string to *map[string]interface{}. This pattern appears in many command files, particularly in the IaaS service commands (volume, backup, network, etc.) when handling label conversions. The code got extracted into util function ConvertStringMapToInterfaceMap and refactored.

Jira Ticket: https://jira.schwarz/browse/STACKITCLI-203

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]

see comments from @marceljk

[marceljk]

Looks good and works well! Can be merged