Support multi-vector retrieval #13257

Techniques like ColBERT represent each Document and query with multiple vectors.

Haystack currently lacks support storing and retrieving multiple vectors. (ColBERT-style reranking is somehow supported).

Before implementing components, I'd suggest investigating the topic and building a POC to explore the overall design.

I imagine something like this:

• find a way to store multiple embeddings in Document
• provide specific embedders (Add Multivector Embedding Components for vLLM Integration haystack-core-integrations#4013)
• support this in at least one Document Store, with specific retrievers

(I'm writing this based on an inital exploration; when we study the topic, additional requirements might emerge)

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