feat: Adding Milvus demo to examples

[franciscojavierarceo]

What this PR does / why we need it:

This pull request introduces several updates and new features to the Feast repository, particularly around the integration of Milvus as an online store. Below is a summary of the key changes:

Documentation Updates

• Updated Overview of Online Stores: Added Milvus to the matrix of supported functionalities in the overview.md file under docs/reference/online-stores/.

New Example Files

• RAG Example: Introduced a new example for RAG (Retrieval-Augmented Generation) under examples/rag/.
  • Added __init__.py.
  • Added README.md.
  • Added feature_repo/ directory containing:
    • __init__.py
    • example_repo.py
    • feature_store.yaml
    • test_workflow.py

Code Updates

• Milvus Online Store Integration:
  • Renamed _get_collection method to _get_or_create_collection in milvus.py.
  • Updated method calls to use _get_or_create_collection instead of _get_collection.
  • Added logic to handle vector fields and search metrics.
  • Improved handling of Proto values and serialization in retrieve_online_documents_v2 method.

Detailed Changes

1. Documentation:

   • Added Milvus to the list of supported online stores in the feature matrix.

2. Examples:

   • Added new example files under examples/rag/.

3. Milvus Integration:

   • Method Renaming: Renamed _get_collection to _get_or_create_collection.
   • Vector Field Handling: Added handling for vector fields and search metrics within the _get_or_create_collection method.
   • Method Updates: Updated the following methods to use _get_or_create_collection:
     • online_write_batch
     • update
     • retrieve_online_documents_v2
   • Proto Value Handling: Improved the handling of Proto values and serialization in retrieve_online_documents_v2.

Which issue(s) this PR fixes:

#4364

Misc

N/A

[dmartinol]

This README seems a generic quickstart guide with no specific connections with the RAG function or the included example.
I assume that it will be more clear once we run it and see what happens, but I believe than a dev experience driven by a notebook may help, WDYT?