Add normalized_cosine vector search similarity

[aprudhomme]

Adds the normalized_cosine similarity as an option for vector search. The usage is the same as cosine similarity, but the vectors are normalized to unit length before indexing. The query vectors are also automatically normalized, allowing use of faster DOT_PRODUCT similarity internally.

The original magnitudes of the indexed vectors are available in the <field_name>._magitude field. This allows for reconstruction of the original vector, if needed.