Add extension hook on `ExecutionPlan` for custom operators to participate in sort requirement pushdown

## Motivation

`sort_pushdown.rs::pushdown_requirement_to_children` in `EnforceSorting` walks down from a `SortExec` and pushes the sort requirement through built-in operators (`UnionExec`, `FilterExec`, projection, etc.). It has no extension point for custom operators.

Custom operators that could benefit from receiving pushed sort requirements, e.g. `OneOfExec` in [datafusion-contrib/datafusion-materialized-views](https://github.com/datafusion-contrib/datafusion-materialized-views) which represents a set of materialized view candidates and needs the requirement stamped as `required_input_ordering` to let cost-based selection differentiate MV candidates that natively satisfy the sort from those that would need a sort wrap, currently can't participate in this walk. Downstream projects have to write their own optimizer rule that duplicates most of upstream's logic.

## Proposal

Add an opt-in method on `ExecutionPlan`:

```rust
/// If this operator can accept a sort requirement from its parent,
/// return the (possibly mutated) operator and the requirement that
/// should be pushed to its children.
///
/// Returning `None` stops the pushdown at this node (equivalent to
/// today's default behavior for non-built-in operators).
///
/// Default returns `None`.
fn try_accept_sort_requirement(
&self,
_requirement: &LexRequirement,
) -> Result, LexRequirement)>> {
Ok(None)
}
```

`pushdown_requirement_to_children` checks this method after the built-in cases and before falling through to a jump. A `OneOfExec`-style operator returns `Some((self.with_required_input_ordering(req), req))` to record the requirement and keep descending.

## Impact

- Custom operators can join the sort-pushdown walk with ~10 LOC per operator instead of a full custom optimizer rule.
- Downstream projects can delete their local sort-pushdown rules once they implement the trait method.
- No behavior change for existing operators (default is opt-in no-op).