Add additional docs in retrieval agent if required

[ShobhitVishnoi30]

Why are these changes needed?

If you have existing documents in the retriever agent instance, and you want to add more documents using the current implementation, it currently replaces the existing ones because the document IDs always start with "doc_0."

To address this, we've updated the functionality to allow assigning different IDs if the user wishes to add additional documents.I've incorporated an "extra_docs" configuration within the retrieval settings. When set to true, the system will assign IDs to the chunks starting from "length+i" when inserting the document into the Chroma database. This modification ensures that existing documents are not replaced, allowing for the addition of more documents to the same collection.

Related issue number

issue

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

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (70cc1f4) 30.27% compared to head (02c46c3) 53.76%.

Files	Patch %	Lines
...ntchat/contrib/qdrant_retrieve_user_proxy_agent.py	71.42%	1 Missing and 1 partial ⚠️
autogen/retrieve_utils.py	33.33%	1 Missing and 1 partial ⚠️
...gen/agentchat/contrib/retrieve_user_proxy_agent.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1028       +/-   ##
===========================================
+ Coverage   30.27%   53.76%   +23.48%     
===========================================
  Files          30       30               
  Lines        3980     3988        +8     
  Branches      897      948       +51     
===========================================
+ Hits         1205     2144      +939     
+ Misses       2696     1648     -1048     
- Partials       79      196      +117     

Flag	Coverage Δ
unittests	53.61% <54.54%> (+23.38%)	⬆️

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[ShobhitVishnoi30]

@sonichi @victordibia @elecnix @halr9000 @joshuavial please review it