Add per-field knn vector format info in SegmentInfo

[tteofili]

When indexing vectors, it is possible to use different vector formats depending on the field; in addition to that it's also possible (although not currently implemented) to have Codecs that can provide different vector formats "dynamically" even for a same field.
To better debug such situations, it would be helpful to have per field vector format information within SegmentCommitInfo (e.g. within the attributes).

This trivial PR adds KnnVectorFormat#name for each field to SegmentInfo#attributes in PerFieldKnnVectorsFormat.
If a doc with field1 is indexed with Lucene99HnswVectorsFormat and a doc with field2 is indexed with Lucene99HnswScalarQuantizedVectorsFormat within the same segment, the correspondingSegmentInfo#attributes will have the following entries:

• "KnnVectorFormat.field1" -> "Lucene99HnswVectorsFormat"
• "KnnVectorFormat.field2" -> "Lucene99HnswScalarQuantizedVectorsFormat"

[ChrisHegarty]

LGTM

[jpountz]

I'm a bit confused: what is the benefit of having it on segment infos in addition to field infos?

[tteofili]

you're right @jpountz , we can probably get away with fieldInfo.getAttribute(PerFieldKnnVectorFormat.PER_FIELD_FORMAT_KEY), I didn't notice that, thanks!