milvus vectordb integration

[AmineDiro]

Solves #146 .

In this PR, I implemented the same interface as the llm-chain-qdrant with some caveats :

• All collection manipulations (ie creation, partitioning, index creation, loading etc...) is left to the user.
• The add_text and add_documents methods return Vec, where string are UUIDs generated by the client in qrant. Milvus has an auto_id FieldSchema, we produce the id generated by Milvus. Arguably we should test that an id field exists :thinking: ?
• qdrant offers a direct payload field, whereas Milvus does not, I implemented the metadata in insert + document content as a VARCHAR field column. The content and metadata are put in a HashMap and sent to Milvus as a string. VARCHAR field have a max_length limit of ~65000. I kept this not to break the API
• search_similarity has the same issue with retrieving and parsing the metadata.

I think that rethinking the VectorStore trait would probably cleanup this implementation (and others ?) a bunch.

[BadMask121]

Hello @AmineDiro i think we need to be able to support new_primary_varchar where we can insert our own primary in varchar instead of auto generated