Support transmux RTC to RTMP. 4.0.95

[winlinvip]

This PR supports:

1. Refine audio transcoder for transcoding aac to opus.
2. Support transcode opus to aac.
3. Support transmux RTC to RTMP.

Why it's so important?

1. RTC DVR, we publish stream by WebRTC. The stream is h.264+opus, so the audio is transcoded to aac, then SRS will DVR to FLV or MP4 file.
2. In a video room, there might be some WebRTC streams, which can be transmux to RTMPs. We can use FFmpeg to transcode and merge RTMP streams, then publish as live. That is transcode a video room to a RTMP stream.
3. Publish by WebRTC and play by RTMP, which use Chrome as a encoder for live broadcasting.

What's now and next?

1. Now this PR supports transmux WebRTC to RTMP.
2. After merged, it's able to DVR WebRTC stream.
3. In the near future, we will write some demos for video room and transmux it to live streaming.

[codecov-commenter]

Codecov Report

Merging #2303 (5ef71f4) into 4.0release (659e173) will decrease coverage by 0.40%.
The diff coverage is n/a.

❗ Current head 5ef71f4 differs from pull request most recent head 19e7e2d. Consider uploading reports for the commit 19e7e2d to get more accurate results

@@              Coverage Diff               @@
##           4.0release    #2303      +/-   ##
==============================================
- Coverage       39.01%   38.61%   -0.41%     
==============================================
  Files             108      108              
  Lines           39914    40386     +472     
==============================================
+ Hits            15572    15594      +22     
- Misses          24342    24792     +450     

| Impacted Files | Coverage Δ | |'

Translated to English while maintaining the markdown structure:

'| Impacted Files | Coverage Δ | |
|---|---|---|
| trunk/src/app/srs_app_async_call.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_bandwidth.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_caster_flv.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_config.cpp | 64.24% <ø> (-1.00%) | ⬇️ |
| trunk/src/app/srs_app_config.hpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_conn.cpp | 46.31% <ø> (ø) | |
| trunk/src/app/srs_app_conn.hpp | 100.00% <ø> (ø) | |
| trunk/src/app/srs_app_coworkers.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_dash.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_dvr.cpp | 0.00% <ø> (ø) | |'

Translated to English while maintaining the markdown structure:

'| trunk/src/app/srs_app_async_call.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_bandwidth.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_caster_flv.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_config.cpp | 64.24% <ø> (-1.00%) | ⬇️ |
| trunk/src/app/srs_app_config.hpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_conn.cpp | 46.31% <ø> (ø) | |
| trunk/src/app/srs_app_conn.hpp | 100.00% <ø> (ø) | |
| trunk/src/app/srs_app_coworkers.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_dash.cpp | 0.00% <ø> (ø) | |
| trunk/src/app/srs_app_dvr.cpp | 0.00% <ø> (ø) | |
| ... and 102 more | |

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data'

Translated to English while maintaining the markdown structure:

'> Δ = absolute <relative> (impact), ø = not affected, ? = missing data

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