feat: implement Error for StunAgentError

[lehmanju]

Fixes #50

[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.39%. Comparing base (23390a3) to head (7ba90e5).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
stun-agent/src/lib.rs	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   96.05%   95.39%   -0.66%     
==========================================
  Files          58       58              
  Lines        2914     2823      -91     
==========================================
- Hits         2799     2693     -106     
- Misses        115      130      +15     

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[sancane]

Thank you for your valuable contribution.