Decimal32/Decimal64 support in Substrait Producer

[matal-nvidia]

Is your feature request related to a problem or challenge?

datafusion-substrait's producer does not support Arrow Decimal32 or Decimal64.
Any DataFusion plan with a Decimal32/Decimal64 field or literal causes
to_substrait_plan to fail with:

NotImplemented("Unsupported cast type: ")
NotImplemented("Unsupported literal: Decimal32(..)")

Describe the solution you'd like

Substrait has one abstract Decimal { precision, scale } and distinguishes
physical widths via type_variation_reference, same mechanism already used for
Decimal256. So:

1. Add DECIMAL_32_TYPE_VARIATION_REF / DECIMAL_64_TYPE_VARIATION_REF in
   variation_const.rs, plus matching entries in Arrow's
   format/substrait/extension_types.yaml.
2. Add Decimal32 / Decimal64 arms in to_substrait_type_from_field and
   to_substrait_literal, mirroring the existing Decimal128 arms.
3. Extend round_trip_types and round_trip_literals with Decimal32(9, 2)
   and Decimal64(18, 2) cases.
4. [optionally] Make consumer/types.rs::from_substrait_type (and the literal consumer)
   branch on type_variation_reference to pick the narrow Arrow width, falling
   back to precision when the anchor is absent.

Describe alternatives you've considered

None

Additional context

No response

[akoshchiy]

take