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)
• Temporarily removed WebArena and AssistantBench. Neither had viable Templates after autogen_magentic_one was removed. Templates need to be update to AgentChat (in a future PR, but this PR is getting big enough already)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.08%. Comparing base (3c30d89) to head (289b166).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5313   +/-   ##
=======================================
  Coverage   78.08%   78.08%           
=======================================
  Files         158      158           
  Lines        9576     9576           
=======================================
  Hits         7477     7477           
  Misses       2099     2099           

Flag	Coverage Δ
unittests	78.08% <ø> (ø)

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[afourney]

I'll add AssistantBench back in another PR once this one's done. This PR was already feeling big, and I was struggling to keep it updated.