feat: sparse embeddings in vector stores

[ludwiktrammer]

Currently, Ragbits supports sparse embeddings (via the SparseVector and SparseEmbedder classes) but none of our vector stores supports this.

1. Change the typing of VectorStoreResult to accept SparseVector (should now be `list[float] | SparseVector)
2. Add support for sparse embeddings to Qdrant and in_memory vector stores. They should be able to take Embedder or SparseEmbedder and then return the vector as list[float] or SparseVector (depending ont he type of embedder).
3. Write HOW-TO documentation on using sparse vectors with vector stores
4. Update the HOW-TO documentation on hybrid search - resolve both the TODO comments that reference sparse vectors.