Core, ORC: surface struct-level null counts for content stats

[bujjibabukatta]

Summary

Proposes one direction for #17463 (open investigation issue, no
consensus yet): let ORC surface struct-level null counts it already
computes, and let V4 content stats store them.

Root Cause

• ORC: ColumnStatistics[] includes real null-count data for
  struct columns, but StatsColumnsVisitor.record() only ever added
  children's field IDs — the struct's own ID was discarded.
• V4 stats: fieldStatsStruct() excluded all nested types
  (struct, map, list) uniformly via isNestedType(), so structs had
  no way to store counts even when a writer could supply them.

Read side is unaffected — InclusiveMetricsEvaluator already does a
generic field-ID lookup with no leaf/struct distinction.

Fix

• OrcMetrics.java — StatsColumnsVisitor.record() now also adds
  the struct's own field ID via ORCSchemaUtil.icebergID(record).
  Bounds unaffected (struct ColumnStatistics matches no
  instanceof branch in fromOrcMin/fromOrcMax).
• StatsUtil.java — fieldStatsStruct() returns a reduced
  value_count/null_value_count-only struct for StructType
  instead of null. Map/list still excluded as before.