Add index to embeddings

What does this MR do and why?

As per https://gitlab.com/gitlab-org/gitlab/-/issues/410581+, we are adding an index to the embeddings.

We changed the distance function to use cosine similarity (cosine) per OpenAI docs.

We refactored the neighbor_for scope, and got rid of select query, since the nearest_neighbors is already returning the nearest and not all the neighbors.

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