feat(storage): add Dell PowerFlex backend support

[jproque-dell]

Add Dell PowerFlex as a new storage backend for Sunbeam.

This introduces a new dellpowerflex backend that enables deploying
OpenStack Cinder with Dell PowerFlex as the block storage provider.
The backend supports both the ScaleIO (PowerFlexDriver) and NVMe-TCP
(PowerFlexNVMeDriver) protocols.

Key configuration options exposed through the PowerFlexConfig model:

• Gateway connection (san_ip, san_login, san_password)
• Storage pool selection (protection_domain:storage_pool format)
• Protocol selection (scaleio or nvme-tcp)
• Gateway port, oversubscription ratio, volume rounding
• Image cache vTree size limits
• REST API timeouts
• Replication device configuration
• SSL certificate, multipath, availability zone

The PowerFlexBackend class integrates with the Sunbeam storage
framework via StorageBackendBase, targeting the
cinder-volume-dellpowerflex charm on the 2026.1/edge channel.

Assisted-By: Devin (Claude)

QA steps

1. Deploy a Sunbeam environment with Cinder storage enabled
2. Configure a Dell PowerFlex backend using sunbeam storage add --backend dellpowerflex
3. Provide the required configuration (san_ip, san_login, san_password, powerflex_storage_pools)
4. Verify the backend is registered and the cinder-volume-dellpowerflex charm is deployed
5. Create a Cinder volume and confirm it is provisioned on the PowerFlex storage pool

Links

N/A