feat: add Proxmox VE provider

[jcarr710]

Closes #6464

Summary

Adds a Proxmox VE provider that monitors infrastructure health by
fetching alerts from the Proxmox REST API.

What's included

• Node status monitoring (offline/degraded → CRITICAL/HIGH alerts)
• VM status monitoring (stopped VMs → HIGH alerts)
• LXC container status monitoring (stopped containers → HIGH alerts)
• Storage pool health (ZFS degraded → HIGH, low space >90% → WARNING)
• Supports all storage types: zfspool, lvmthin, dir, lvm
• API token authentication (user@realm!tokenname format)
• Configurable SSL verification
• Session-level retry strategy for resilience

Tested

Tested against a live Proxmox VE 9.1 instance. All endpoints
confirmed working: version, nodes, qemu, lxc, storage.

Auth config

• host: Proxmox URL (e.g. https://192.168.1.100:8006)
• token_id: API token ID (user@realm!tokenname)
• token_secret: API token secret
• verify_ssl: Whether to verify SSL certificates (default: False)

API reference

https://pveapi.proxmox.com/

[CLAassistant]

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