Add PTZ support for Xiaomi cameras via `xiaomi://` protocol exposed as ONVIF

go2rtc already handles Xiaomi cameras for streaming via xiaomi://. The next step would be to support PTZ commands through the same protocol.

The ideal flow would be:

1. go2rtc sends PTZ commands using the native Xiaomi API
2. go2rtc exposes them locally as ONVIF
3. NVR like Frigate can then point their ONVIF config to go2rtc as a proxy

This would make go2rtc a full proxy for Xiaomi cameras. Consistent with the existing architecture, and would benefit anyone using Frigate or any ONVIF-based NVR with a Xiaomi PTZ-capable camera (e.g. C300).