Turn agent_eval into a proper module to include it in Python package

Without __init__,py it seems `agent_eval` directory is not included in pyautogen Python package.

So the only way to use `agent_eval` is via cloning the repo and adding it to the `sys.path`, which is awkward.

autogen/agentchat/contrib/agent_eval/__init__.py

@@ -0,0 +1 @@
+