Implement SPED (STUN Protocol for Embedding DTLS)

[fippo]

(based on #914 )

Part of pion/webrtc#3335

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.26%. Comparing base (6e0640f) to head (95838f7).

Files with missing lines	Patch %	Lines
piggyback.go	77.67%	21 Missing and 4 partials ⚠️
agent.go	80.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #876      +/-   ##
==========================================
- Coverage   88.44%   88.26%   -0.19%     
==========================================
  Files          45       46       +1     
  Lines        5886     6014     +128     
==========================================
+ Hits         5206     5308     +102     
- Misses        469      491      +22     
- Partials      211      215       +4     

Flag	Coverage Δ
go	88.26% <80.00%> (-0.19%)	⬇️

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[cnderrauber]

Thank you for this great feature! I have integrated this into the livekit SFU and made some connectivity checks, found 2 regression cases:

1. The ice-lite agent doesn't ping the remote actively, so the piggyback packets will be sent in response to the remote peer's connectivity check, cost longer than the non-sped case. Fixing this could conflict with spec, maybe we can mark it as an exception.
2. When a remote peer sends a done signal after the agent is connected while still having inflight piggyback packets, it will mark the state to complete and doesn't have a chance to flush the packets. And also the Piggyback() will swallow all dtls packets but return true in this state (complete after connected).

Both are fixed in our fork livekit@85afccd, livekit@5eec925. Will merge them back to upstream once this PR is merged if they make sense.