Default `bbq` vector indices to oversample

[benwtrent]

Description

We know to achieve adequate recall in a distributed vector index with bbq oversampling and rescoring is required. Let's have that on by default for users.

Benchmarking shows that a default around 3 gives a nice balance of query latency and recall.

Of course, this should be overridable at query time, and if the user wants to provide their own oversampling defaults, they should be able to do that at the mapping level.

Related: #124581

[elasticsearchmachine]

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