Storage Layer Abstraction

[noctarius]

This pull request introduces a storage layer abstraction which removes the hard dependency on simplyblock and enables alternative underlying storage engines, such as LVM.

What Changed

1. Moved simplyblock-specific logic behind backend modules

• Moved StorageClass manifest build logic out of generic deployment flow into simplyblock backend module.
• Moved simplyblock credential loading and volume identifier resolution to simplyblock boundary code.
• Refactored branch DB volume size resolution and IOPS updates to call backend abstractions (lookup_volume, update_performance) instead of direct simplyblock API calls.

2. Backend-driven clone/restore orchestration

• Branch clone/restore flows in branch API now call backend operations rather than assembling snapshot/storage class details directly.
• Added backend operations for clone/restore execution and snapshot usage retrieval.

3. Snapshot/storage class de-hardcoding

• Removed hardcoded branch snapshot class path.
• backup.py and backupmonitor.py now resolve snapshot class via active backend.
• Chart defaults no longer hardcode simplyblock-csi-sc (storageClassName defaults set to empty and populated by controller).

4. Capability-aware IOPS behavior

• DeploymentParameters.iops is now optional.
• Clone validation requires IOPS only when backend supports volume IOPS controls.
• Resize IOPS updates are capability/policy gated:
  • supported backend: apply runtime update
  • unsupported + strict: fail
  • unsupported + best_effort: no-op runtime update, persist logical allocation
• Branch persistence remains compatible via fallback/default (IOPS_MIN) where needed.

5. Resource/metering updates

• Resource usage collection routes through backend usage providers (no direct simplyblock calls in resources API).
• Snapshot used-size calculation is backend-provided.
• volume_metrics_available now initializes from backend metric capabilities and flips false on runtime collection failure (not hardcoded true).

6. Config and API surfacing

• Added missing deployment settings:
  • storage_default_class
  • storage_snapshot_class
• /system/resource-limit-definitions is backend-aware: iops definition is only returned when backend supports volume IOPS.