Support file stats on nested fields

[asubiotto]

Currently, when writing file statistics, only top-level field stats are stored in the vortex file footer:

vortex/vortex-layout/src/layouts/file_stats.rs

Line 45 in 191228e

/// Note: for now this only collects top-level struct fields.

Pruning on nested fields does work at a chunk level I believe, so this is specifically about file level stats to allow pruning whole files where possible.