Utilize AcceptDocs interface for lazy evaluation of filters for `*_flat` vector indices

Description

We currently have higher level logic to lazily evaluate the filter for our flat codecs. However, with the new AcceptDocs interface, it seems to me we should be able to delegate that down to our flat formats. This way they can simply utilize the AcceptDocs iterator when scoring instead of fully realizing the bitset (which can be truly costly).

I foresee this being split between multiple PRs:

• First, utilizing accept docs for lazy iteration within our custom flat formats.
• Second, removing the custom logic and verifying with benchmarks that performance is still acceptable. (this will require running with the https://github.com/elastic/rally-tracks/tree/master/random_vector track I think)