Consider renaming `AggregateUDFImpl::is_ordered_set_aggregate` to `AggregateUDFImpl::supports_within_group_clause`

[Jefffrey]

Is your feature request related to a problem or challenge?

See #17805 (comment)

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

Could make hypothetical-set aggregate functions easier in future: https://www.postgresql.org/docs/9.4/functions-aggregate.html#FUNCTIONS-HYPOTHETICAL-TABLE