Significant updates to agbench.

[afourney]

• Updated HumanEval template to use AgentChat
• Update templates to use config.yaml for model and other configuration
• Read environment from ENV.yaml (ENV.json still supported but deprecated)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.09%. Comparing base (756e2a4) to head (db6964b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5313      +/-   ##
==========================================
- Coverage   76.10%   76.09%   -0.02%     
==========================================
  Files         157      157              
  Lines        9475     9475              
==========================================
- Hits         7211     7210       -1     
- Misses       2264     2265       +1     

Flag	Coverage Δ
unittests	76.09% <ø> (-0.02%)	⬇️

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