New Retriever API

[AyushExel]

I communicated these changes over discord

Why are these changes needed?

This PR creates a new Retriever API that makes plugging different retrievers simpler. It also removes hardcoding of particular vectorDBs and makes a centralized module that deal with optional vectordb dependencies

Retrieval functions were hardcoded. This PR introduces a new Retriever abstract interface that simply has 3 functions.
Retriever.init_db() - To initialize connection/client or nothing if in serverless setting
Retriever.ingest_data() - To upsert data in the db
Retriever.query() - To run queries
Any DB provider can simply implement these functions and register as a supported retriever for Autogen.
Note: This PR will not break the existing examples/notebooks.
This is still WIP but I tested the current chromadb implementation and it works well with the existing examples.

Related issue number

Closes #586 #416 and a few others

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.

[thinkall]

Thank you @AyushExel , I agree with the design you proposed. Users will be able to use different vector dbs by simply giving different keys.

Should we put "retriever" folder in "contrib" as well? Maybe even move retrieve_utils.py to retriever folder.

What do you think? @sonichi