UDPMuxSink for UDPMux allows to inspect packets before being dropped

[astroza]

Description

This is one of the features that helped us to build a distributed version of Pion.

Inspecting UDPMux discarded packets makes it possible to evaluate and restore a PeerConnection if needed

[nils-ohlmeier]

Looks good to me.
But someone else should probably review this as well.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.50%. Comparing base (35bb3fb) to head (589aef2).

Files with missing lines	Patch %	Lines
udp_mux.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
- Coverage   78.69%   78.50%   -0.20%     
==========================================
  Files          41       41              
  Lines        4769     4773       +4     
==========================================
- Hits         3753     3747       -6     
- Misses        783      790       +7     
- Partials      233      236       +3     

Flag	Coverage Δ
go	78.50% <25.00%> (-0.20%)	⬇️
wasm	22.45% <0.00%> (-0.02%)	⬇️

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[Sean-Der]

@nils-ohlmeier You can merge anything without me! As long as it passes the tests I don't care. Nothing worse then waiting on a slow maintainer :)

@astorza that's really cool you were able to build a distributed version! Would you mind writing a test for this? Just assert that if set it really does get fired.

thank you so much for the contribution