This patch adds vector index options allowing to enable quantization and oversampling.
Specific quantization value will be used internally by vector store.
In the current implementation, `get_oversampling` allows us to decide how many times more candidates
to retrieve from vector store - final response is still trimmed to the given limit.
It is a first step to allow rescoring - recalculation of similarity metric and re-ranking.
Without rescoring oversampling will be also further optimized to happen internally in vector store.
Fixes https://scylladb.atlassian.net/browse/SCYLLADB-82
Ref https://scylladb.atlassian.net/browse/SCYLLADB-83
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