Add plugins for vector stores

[mocobeta]

emcli currently uses Chroma as the default vector store.

This issue aims to add support for other popular embedded vector stores or vector database servers, such as:

• Qdrant
• Elasticsearch
• Milvis
• Weaviate
• PGVector
• Faiss
• sqlite-vss

[mocobeta]

As of embcli-core v0.1.2, the bundled vector store is LanceDB (#79). Chroma integration is provided by a dedicated plugin (#80).