Add DTLS KeyLog configuration option in WebRTC API

[npmenard]

Description

Add an option in the setting engine to log TLS key material when a DTLS connection is established with a peer. The option exists in pion/dtls but is not easily accessible. A user would use it very similarly to the crypto/tls package :

...
if keyLog := os.Getenv("SSLKEYLOGFILE"); len(keyLog) != 0 {
		w, _ := os.OpenFile(keyLog, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0600)
		settingEngine.SetDTLSKeyLogWriter(w)
}
...

This is somewhat related to pion/interceptor#251 as it lets a user use Wireshark or a similar tools to capture and decrypt a particular session.

[edaniels]

Nice! Just one change to make.

[edaniels]

Suggested change

	// Logging key material compromise security and should only be use for debugging.
	// Logging key material compromises security and should only be use for debugging.

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -0.24% ⚠️

Comparison is base (457ff6c) 77.83% compared to head (b969610) 77.59%.

❗ Current head b969610 differs from pull request most recent head dfce290. Consider uploading reports for the commit dfce290 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    pion/webrtc#2522      +/-   ##
==========================================
- Coverage   77.83%   77.59%   -0.24%     
==========================================
  Files          87       87              
  Lines        9357     9360       +3     
==========================================
- Hits         7283     7263      -20     
- Misses       1646     1665      +19     
- Partials      428      432       +4     

Flag	Coverage Δ
go	79.35% <33.33%> (-0.27%)	⬇️
wasm	70.15% <ø> (ø)

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Files Changed	Coverage Δ
settingengine.go	50.00% <0.00%> (-0.95%)	⬇️
dtlstransport.go	63.69% <100.00%> (+0.10%)	⬆️

... and 3 files with indirect coverage changes

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[Sean-Der]

This is really cool @npmenard

If you are interested I would like to make a post on using this! Debugging WebRTC is hard, this is a lot easier then the alternative.