Support for 2-way audio over RTSP back channel, ONVIF Profile T

[felipecrs]

This is the closest we have for an standard for 2-way audio communication for cameras. It's known as the RTSP back channel, documented by the ONVIF Profile T: https://www.onvif.org/specs/stream/ONVIF-Streaming-Spec.pdf

Many Dahua and Hikvision cameras of the past few years have this feature in their stock firmware, and go2rtc as an end-user application for consuming/re-streaming cameras streams has added first class support for it.

Even Frigate (a modern and open-source NVR software) is adding go2rtc to its core, thus bringing 2-way audio support as well through the RTSP back channel.

But since many cameras lack this feature in their stock firmware, if OpenIPC (Majestic) had support for it this would be a big plus (at least for me) for switching the firmware.

[widgetii]

It should be implemented in smolrtsp library first and then will be appeared in Majestic itself naturally. PRs or sponsors are welcome