FAISS in OpenSearch: Support HNSW for cosine

[tstadel]

As a user I want to use FAISS's HNSW for cosine similarity too with OpenSearchDocumentStore.

According to https://opensearch.org/docs/latest/search-plugins/knn/knn-index/ FAISS plug-in only supports dot product and l2. So we have to normalize vectors before indexing/searching.

• [ ] normalize vectors manually at index time
• [ ] normalize vectors manually at query time
• [ ] use dot_product space under the hood