Support group_concat function

[jovezhong]

Reference https://clickhouse.com/docs/en/sql-reference/aggregate-functions/reference/groupconcat

The use case is to follow the ClickHouse docs script to generate the markdown based on settings source code.

SELECT prefix || (SELECT groupConcat(*) FROM main_content)
INTO OUTFILE 'docs/en/operations/settings/settings.md' TRUNCATE FORMAT LineAsString

There is only 1 column in CTE main_content, so it uses groupConcat(*)

I can use group_array(col) to create an array but there is no function to turn this array of string to a single string. concat function takes multiple string params, but not an array.

[jovezhong]

One workaround I found is array_fold(acc,x->concat(acc,x),group_array(str),''), but better to just use group_concat(str) when this is available.