feat: Extract NDV (distinct_count) statistics from Parquet metadata

This change adds support for reading Number of Distinct Values (NDV)
statistics from Parquet file metadata when available.

Previously, `distinct_count` in `ColumnStatistics` was always set to
`Precision::Absent`. Now it is populated from parquet row group
column statistics when present:

- Single row group with NDV: `Precision::Exact(ndv)`
- Multiple row groups with NDV: `Precision::Inexact(max)` as lower bound
  (we can't accurately merge NDV since duplicates may exist across
  row groups; max is more conservative than sum for join cardinality
  estimation)
- No NDV available: `Precision::Absent`

This provides foundation for improved join cardinality estimation
and other statistics-based optimizations.

Relates to #15265

Author: asolimando