Rename retrieve chat

[thinkall]

Why are these changes needed?

Address issue #23

The current naming is not perfect, it would be better to use RAG rather than Retrieve. My only concern is that this PR would be a breaking change to the RetrieveChat APIs.

Waiting till stable, then maybe move from contrib to core and rename it.

Related issue number

Closes #23

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

Merging #147 (7b79fda) into main (5ff85a3) will not change coverage.
The diff coverage is 9.37%.

@@           Coverage Diff           @@
##             main     #147   +/-   ##
=======================================
  Coverage   41.03%   41.03%           
=======================================
  Files          17       17           
  Lines        2091     2091           
  Branches      469      469           
=======================================
  Hits          858      858           
  Misses       1156     1156           
  Partials       77       77           

Flag	Coverage Δ
unittests	40.93% <9.37%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
autogen/rag_utils.py	63.69% <ø> (ø)
autogen/agentchat/contrib/rag_assistant_agent.py	33.33% <66.66%> (ø)
autogen/agentchat/contrib/rag_user_proxy_agent.py	4.41% <3.44%> (ø)

[gagb]

@pcdeadeasy this PR was to address your feedback on a previous PR. Please review it or if its not a big deal close the issue and the PR -- its 2 weeks old now.

#23