feat(cli): onboard iaas volume backup api

[Benjosh95]

Description

Onboarding the volume backup API https://docs.api.stackit.cloud/documentation/iaas/version/v1#tag/Backups to manage the backup resources via the cli.

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)

Testing-Instructions

1. Create a test volume to have something to backup
   stackit volume create --name backup-test-vol --size 10 --performance-class storage_premium_perf0 --availability-zone eu01-1 --labels initial=true,purpose=backup-testing

2. Create backup of the volume with name and labels
   stackit volume backup create --source-id <VOLUME_ID> --source-type volume --name first-backup --labels state=original,test=true

3. List backups with limit and label selector
   stackit volume backup list --limit 5 --label-selector state=original

4. Update backup with new name and labels
   stackit volume backup update <BACKUP_ID> --name updated-backup --labels state=modified,test=true

5. Describe backup to verify changes
   stackit volume backup describe <BACKUP_ID>

6. Restore backup
   stackit volume backup restore <BACKUP_ID>

7. Clean up by deleting backup
   stackit volume backup delete <BACKUP_ID>

8. Clean up by deleting volume
   stackit volume delete <VOLUME_ID>

[rubenhoenle]

@Benjosh95 please check the failing CI pipeline

[Benjosh95]

@Benjosh95 please check the failing CI pipeline

CI pipeline fails due to missing/undefined waiters that will be merged with this PR: stackitcloud/stackit-sdk-go#2669