Add duration utils

[RobertRuf]

Description

Duration was added to the core utils package and includes utilities for:

• Duration parsing and conversion with flexible unit support
• Time-based calculations including calendar-aware month handling
• Configurable validation with minimum/maximum boundaries

Checklist

• 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
• 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)

[Fyusel]

Thank you for contribution this. Looks good!

[marceljk]

Thanks for your contribution! It's really well documented and tested 😃