agent: support arbitrary STUN request timeouts

[ystreet]

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[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.59%. Comparing base (c106a15) to head (330baee).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
stun-proto/src/agent.rs	85.71%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   87.65%   87.59%   -0.06%     
==========================================
  Files          19       19              
  Lines        1579     1596      +17     
==========================================
+ Hits         1384     1398      +14     
- Misses        195      198       +3     

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