FAISS in OpenSearch: Support Product Quantization

[tstadel]

As a user I want to use FAISS's product quanitzation feature with OpenSearchDocumentStore.

According to https://opensearch.org/docs/latest/search-plugins/knn/knn-index/ FAISS plug-in has two param for pq:

• m: number of sub vectors to split the original vector
• code_size: the number of bits to encode a sub-vector into

We only support setting m as code_size is only relevant for IVF, static for HNSW.

• [ ] add param product_quantization_subvectors with default value None
• [ ] if product_quantization_subvectors is not None, set pq params with m=product_quantization_subvectors
• [ ] add check whether existing index has requested setting