Add PayloadToMimeType map to StreamInfo

[5ur3]

Description

This issue arose during the implementation of a RED FEC decoder interceptor. A sender can announce both an original codec (Opus, in this example) and RED in the SDP as follows:

a=rtpmap:111 opus/48000/2
a=fmtp:111 minptime=10;useinbandfec=1;usedtx=1
a=rtpmap:63 red/48000/2
a=fmtp:63 111/111

Following this announcement, the sender may or may not use RED for audio transmission. Furthermore, the sender can dynamically switch between unprotected Opus and RED-protected streams during a session.

Currently, the receiver's interceptor has no way to determine the MIME type of a received packet. The StreamInfo structure only contains the capabilities of the first negotiated codec for the media section, which is not always the codec being used for transmission.

This PR resolves this ambiguity by adding a PayloadToMimeType map to StreamInfo. This allows decoder interceptors to match a received packet's payload type to its corresponding MIME type. For instance, with this change, the RED FEC decoder interceptor can now reliably differentiate between unprotected audio packets and RED-protected audio packets.

Implementation PR

The PayloadToMimeType field is set within the pion/webrtc repository. The work is handled by the following pull request: pion/webrtc#3175

This corresponding PR is currently a draft because it depends on the changes from this PR. Once this is merged, the pion/webrtc PR will be updated with a commit that bumps its dependency on this repository to include these changes, and it can then be finalized for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.46%. Comparing base (7be3ed2) to head (0b40e43).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #346      +/-   ##
==========================================
+ Coverage   76.00%   78.46%   +2.45%     
==========================================
  Files          81       81              
  Lines        5130     5130              
==========================================
+ Hits         3899     4025     +126     
+ Misses       1042      930     -112     
+ Partials      189      175      -14     

Flag	Coverage Δ
go	78.46% <ø> (?)
wasm	76.00% <ø> (ø)

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[JoeTurki]

Hello @5ur3 Thank you for the work you do, Let me know when you need review for this PR, also I invited you to the org so you can make branches directly, and run the CI without waiting for approvals.

[5ur3]

@JoeTurki Thanks! This PR is ready to be reviewed

[JoeTurki]

Hello, I added a comment in the other PR, maybe there is a way to handle this using another fix? pion/webrtc#3175 (review)

[5ur3]

Let's continue the discussion in pion/webrtc#3175 (review)