AI: WebRTC: Support optional msid attribute per RFC 8830. v7.0.126 (#4570) (#4572)

Fix issue #4570 by supporting optional `msid` attribute in WebRTC SDP
negotiation, enabling compatibility with libdatachannel and other
clients that don't include msid information.

SRS failed to negotiate WebRTC connections from libdatachannel clients
because:
- libdatachannel SDP lacks `a=ssrc:XX msid:stream_id track_id`
attributes
- SRS required msid information to create track descriptions
- According to RFC 8830, the msid attribute and its appdata (track_id)
are **optional**

If diligently look at the SDP generated by libdatachannel:

```
a=ssrc:42 cname:video-send
a=ssrc:43 cname:audio-send
```

It's deliberately missing the `a=ssrc:XX msid:stream_id track_id` line,
comparing that with this one:

```
a=ssrc:42 cname:video-send
a=ssrc:42 msid:stream_id video_track_id
a=ssrc:43 cname:audio-send  
a=ssrc:43 msid:stream_id audio_track_id
```

In such a situation, to keep compatible with libdatachannel, if no msid
line in sdp, SRS comprehensively and consistently uses:

* app/stream as stream_id, such as live/livestream
* type=video|audio, cname, and ssrc as track_id, such as
track-video-video-send-43

.vscode/README.md

@@ -40,6 +40,11 @@ cmake --build $HOME/git/srs/trunk/cmake/build
 
 ## macOS: SRS UTest
 
+The most straightforward way is to select a test name like `WorkflowRtcManuallyVerifyForPublisher`,
+then select `Debug gtest (macOS CodeLLDB)` and run the debug.
+
+Or you can use the following way to run specified test from the test panel.
+
 Install the following extensions:
 
 - C++ TestMate