Support two indexing styles: ColBERT/PLAID style optimisation and HNSW-style uncompressed indexes

[bclavie]

Currently, we only use the ColBERT optimised indexes, or index-free in-memory encodings.

For low-to-medium volumes of documents, not using the ColBERT optimisation can have advantages: easier CRUD, potentially better performance, the concept is easier to grasp, quicker indexing time... (and tradeoffs: much bigger footprint, no constant-time querying, etc...)

We should provide the ability to create both kinds of indexes, and let the user decide which indexing style they wish to use.

[0-hero]

Are you looking to use something like FAISS or https://github.com/nmslib/hnswlib ?