Add filter pushdown AST implementation

Implements Phase 2 of filter pushdown support:

- Add vgi/table_filter_pushdown.py with typed AST classes:
  - Filter base class with evaluate() method
  - ConstantFilter, InFilter, IsNullFilter, IsNotNullFilter
  - AndFilter, OrFilter for compound expressions
  - StructFilter for nested field access
  - PushdownFilters container with helper methods
  - SQL generation via to_sql()
  - Column bounds extraction via get_column_bounds()

- Add pushdown_filters property to TableFunctionBase (cached_property)
- Add auto_apply_filters Meta flag support to TableFunctionGenerator
- Add auto_apply_filters Meta flag support to TableInOutFunction
- Add create_pushdown_filters() test utility
- Add comprehensive tests (48 new tests)
- Export key classes from vgi package

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rustyconover