RTC: Support OPUS stereo SDP option. v5.0.203 v6.0.105 (#3910)

In an SDK that supports RTC Opus stereo, the parameter "stereo=1" may
appear. SRS (Spatial Reference System) needs to handle this correctly
and return an answer to enable WebRTC stereo support.



---------

`TRANS_BY_GPT4`

trunk/src/app/srs_app_rtc_source.hpp

@@ -389,11 +389,13 @@ class SrsAudioPayload : public SrsCodecPayload
     {
         int minptime;
         bool use_inband_fec;
+        bool stereo;
         bool usedtx;
 
         SrsOpusParameter() {
             minptime = 0;
             use_inband_fec = false;
+            stereo = false;
             usedtx = false;
         }
     };