Fix relay candidate preference

[amanakin]

Description

This fixes the handling of relay preferences. Previously, LocalPreference for CandidateRelay was not being called, causing all relay protocols to have the same priority. This change introduces protocol-based preferences (UDP: 3, DTLS: 2, TCP: 1, TLS: 0).

Reference issue

Fixes #879

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.49%. Comparing base (3a1823b) to head (e72e16b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #880      +/-   ##
==========================================
+ Coverage   88.36%   88.49%   +0.13%     
==========================================
  Files          43       43              
  Lines        5509     5512       +3     
==========================================
+ Hits         4868     4878      +10     
+ Misses        447      440       -7     
  Partials      194      194              

Flag	Coverage Δ
go	88.49% <100.00%> (+0.13%)	⬆️

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[JoTurk]

@amanakin thank you so much, I'm reviewing this right now.

I see that contribute to pion frequently, would you like an invite to the org, no exceptions and nothing heavy-weight is required :) but we would like your reviews and it will let you run the CI yourself.

[amanakin]

@JoTurk Hey! Sure, happy to help. :) Regarding this PR, I've started a thread in discord for discussion. libwebrtc's preference for DTLS looks weird..

[JoTurk]

yeah i was reading the discord discussion, and libwebrtc impl.
sent the invite :)

[JoTurk]

code is okay, not super sure about the behavior change and preference. but it seems fine and it makes sense to match libwebrtc.

[fippo]

pion can probably change the specific local type preference without much issue. These days we have "relayProtocol" exposed on candidates so folks should look for that instead of reverse-engineering as was required in the old days

[amanakin]

Well, looks like the webrtclib maintainers agree that DTLS should have lower priority than UDP
https://webrtc-review.googlesource.com/c/src/+/443160

[JoTurk]

looks good for me, i think you should wait few days for other inputs :)