Implement distinct and order by clause for string_agg aggregate function #8260

Is your feature request related to a problem or challenge?

After #8148 merge, we have implemented the basic string_agg function, but order by and distinct are not supported yet.

As @alamb said, if there is no order by clause, the string_agg output result is non deterministic , so it is very important to support order by in string_agg.

One thing I thought of was is I think this function will be non deterministic when aggregating without an ORDER BY -- specifically since the aggregate can see the inputs in any order when doing a multi-phase repartitioned group by.

Describe the solution you'd like

add support of distinct and order by for string_agg function.

Describe alternatives you've considered

No response

Additional context

No response