feat(sdk): add linter and fix linter findings

[rubenhoenle]

Description

relates to STACKITSDK-219

Checklist

• Issue was linked above
• No generated code was adjusted manually
• Changelogs
  • Changelog in the root directory was adjusted
  • Changelog(s) of the service(s) were adjusted
• 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)