FAISS in OpenSearch: Support HNSW for dot product and l2

[tstadel]

As a user I want to use FAISS's HNSW ANN search engine in OpenSearchDocumentStore in order to compare it against nmslib.

According to https://opensearch.org/docs/latest/search-plugins/knn/knn-index/ FAISS plug-in only supports dot product and l2 space. So we exclude any other similarity type here.

• [ ] add param knn_engine with default "nmslib" to construtor
• [ ] set index mappings: use the same hnsw params as for nmslib
• [ ] throw on other similarity types