RTMP edge support transmux RTMP to RTC

[winlinvip]

In #2200, the functionality of converting RTMP to RTC in RTMP Edge is mentioned separately in this branch and PR.

Due to the complex relationship between Edge and RTC, it needs to be considered separately.

Regarding why RTC only works in Origin mode, I have had a detailed discussion with Zhihong. In simple terms, Edge is a design for RTMP live clusters, while RTC is a cascading cluster that does not use the Edge method to implement RTC cascading.

HLS cannot achieve this because it cannot trigger Edge to fetch slices when HLS is being played, and the client cannot wait for it.

RTC can achieve this but it is not reasonable. When playing or pushing RTC, triggering Edge to fetch sources will turn RTC into internal transmission of RTMP, which does not meet the latency requirements. Considering RTC pushing, Edge fetches sources and pushes them to RTMP, then another edge plays RTC (which fetches sources from RTMP). In this way, it becomes:

RTC pushing => RTMP Edge fetches sources => RTMP Origin server receives the stream => RTMP Edge plays => RTMP converts to RTC

This essentially involves RTC cascading through RTMP Edge, going back and forth, which is not only complex but also fails to achieve the business objectives.

During playback, it is worth considering that Edge supports RTC. The benefits are as follows:
1. In the live streaming system, it is possible to switch to RTC in the last hop, which can be used for low-latency live streaming at a certain scale.

The issues are as follows:
1. It is quite complex to trigger RTMP backhaul using RTC.
2. If RTC has its own cascading mechanism, there is no need to use the RTMP system.

However, it is unrelated to this RTC to RTMP conversion. I have removed the RTC streaming and Edge code for now. If we need to support RTC playback triggering Edge backhaul in the future, we can consider it separately.

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