Support bbq quantization for `byte` vectors

[benwtrent]

Description

bbq_* indices quantize vectors to lower number of bits. We should allow quantizing element_type: byte vectors to lower bit levels (1, 2, 4, etc.).

The tricky part comes around with clustering, but even here, I would assume when clustering, we will ensure that the centroids are stored as byte[] as well, this way we still gain (whatever possible) SIMD comparisons between byte[], byte[] and don't have to score float[], byte[] for quantization.

Also remember, that element_type: byte is signed bytes (this might not play a factor, but its important to keep in mind).

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)