Finish lite-lite candidate selection

[JoTurk]

Description

While testing pion/ice@main against WebRTC, I noticed that TestPeerConnection_IceLite was failing. Pion supports ICE-lite to ICE-lite connections in both the controlling and controlled roles, but it did not correctly handle pair selection with multiple candidates #959 and #961
ICE-lite agents do not send connectivity checks, so the previously selected pair could remain selected even when a better candidate pair became available later. ICE-lite behavior is defined across several sections of RFC 8445, particularly Section 6.2. Candidate priority uniqueness is required by Section 5.1.2.
This change assigns unique priorities to locally gathered ICE-lite candidates and re-evaluates the standard ICE pair-priority ordering as candidates arrive. Both peers can select the same best pair and switch when a higher-priority pair becomes available, without sending connectivity checks.

Tldr: this fixes a race where both agents select different candidates because of the lack of connectivity checks.

[codecov]

Codecov Report

❌ Patch coverage is 69.64286% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.09%. Comparing base (995396d) to head (42c6883).

Files with missing lines	Patch %	Lines
agent.go	64.44%	11 Missing and 5 partials ⚠️
candidate_base.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #968      +/-   ##
==========================================
- Coverage   88.31%   88.09%   -0.23%     
==========================================
  Files          46       46              
  Lines        6368     6408      +40     
==========================================
+ Hits         5624     5645      +21     
- Misses        510      522      +12     
- Partials      234      241       +7     

Flag	Coverage Δ
go	88.09% <69.64%> (-0.23%)	⬇️

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