Refactor: extract sort pushdown logic from FileScanConfig into separate module

[zhuqi-lucas]

Is your feature request related to a problem or challenge?

FileScanConfig in datafusion/datasource/src/file_scan_config.rs has grown large after the sort pushdown optimization (#21182) added statistics-based file sorting, non-overlapping validation, and NULL handling logic.

As noted by @alamb in #21182 (comment):

As a follow on PR it might be nice to figure out how to move some of this code out of FileScanConfig and into some other smaller module

Describe the solution you'd like

Extract sort pushdown related code from FileScanConfig into a dedicated module, e.g. datafusion/datasource/src/sort_pushdown.rs:

• try_pushdown_sort()
• rebuild_with_source()
• try_sort_file_groups_by_statistics()
• sort_files_within_groups_by_statistics()
• any_file_has_nulls_in_sort_columns()
• Related helper functions and types (SortedFileGroups, etc.)

This is a pure refactor — no behavior changes.

Related issues:

• [EPIC] Sort pushdown / partially sorted scans #17348 — Parent issue: sort pushdown optimization
• feat: sort file groups by statistics during sort pushdown (Sort pushdown phase 2) #21182 — Sort pushdown phase 1 (where the code was added)