Vector format info from FieldInfo in Index Segments API

[tteofili]

Engine should collect KnnVectorFormat from FieldInfo instead of Codec. This will result in reading actual per-segment, per-field vector format information; in fact right now vector format information is provided by codec, which in turn is read from mappings, and therefore not always reflecting what's on-disk (e.g. when updating dense vector mappings).

[tteofili]

I had initially opened a Lucene PR to add this information in SegmentInfo, but that same information is present in FieldInfo and hence I've closed it and opened this PR which makes things much simpler.

[elasticsearchmachine]

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