Allow different bit sizes for `bbq` indices

[benwtrent]

Description

bbq_* indices right now only quantize to a single bit. We should allow quantizing to different bit levels.

I would say the first couple should be 2, 4, 7. Additionally, we need to determine if keeping the asymmetric nature of the query bit size vs. doc bit size is still valuable at higher bit sizes.

For example, should we quantize queries to 6 bits when docs are 2 bits? 7 for 4?

[elasticsearchmachine]

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