Add a dedicated physical expression for struct-aware casting that mirrors the existing `cast_column` helper semantics.

[kosiew]

1. Introduce CastColumnExpr expression node

• Goal: Add a dedicated physical expression for struct-aware casting that mirrors the existing cast_column helper semantics.

• Scope & Location: Implement in datafusion/physical-expr (expression definition, evaluation logic, scalar/array handling) and expose via the crate’s public API.

• Key Tasks:

  • Define the CastColumnExpr struct with schema metadata (input field, target field) and implement PhysicalExpr.
  • Use datafusion_common::nested_struct::cast_column in evaluate for array and scalar cases, preserving nullability and field ordering.
  • Ensure type coercion rules and data_type / nullable methods reflect the target field accurately.
  • Add unit tests covering basic casts, missing child fields, null-padding, and nested structs.
  • Update documentation or module comments describing when to use CastColumnExpr.

• Out of Scope: Any planner/optimizer integration, protobuf wiring, or schema-rewriter changes.