This commit adds coster tests for distinct-on expressions. There were no
such tests before.

Release note: None

Limit hints are now considered when costing DistinctOn expressions,
similarly to streaming GroupBy expressions. This matches the behavior of
the execution logic, where a DistinctOn emits a row with distinct
grouping columns as soon as it is read from its input. It does not need
to read all of its input to begin emitting rows.

Release note (performance improvement): The optimizer now costs
distinct-on operators more accurately. It may produce more efficient
query plans in some cases.

The `optimizer_use_improved_distinct_on_limit_hint_costing` session
setting has been added which enables the improved costing of DistinctOn
expressions with limit hints added in the previous commit. It is enabled
by default.

Release note: None