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.

[joseph-isaacs]

We can store the stats for all element on a post-order walk of the dtype there each field

{name=utf8?, age=i32} goes to [name stats_set, age stats_set]

{name=utf8?, age=i32}? goes to [name stats_set, age stats_set, struct stats_set (null count)]

Then we would need to extend the the stats set footer with an optional/default=false bool is_nested which if false was original behaviour and if true will have the post-order walk.

Open question:

How do we store list stats?