Add CONCAT_WS function (enabled in MSSQL, MySQL, Postgres libraries)

This function returns a string resulting from the concatenation, or joining, of two or more string values in an end-to-end manner. It separates those concatenated string values with the delimiter specified in the first function argument. It has the following syntax:

CONCAT_WS ( separator, argument1, argument2 [, argumentN]... )

Unlike CONCAT function, CONCAT_WS ignores NULL value, for example:

select concat_ws(',', 'a', 'b', null, null, 'c');
-- result in 'a,b,c'

One important thing to be noted is that CONCAT_WS has two NULL semantics for the separator argument in different Database products(So we need two function in Calcite to represent).

Taking the following SQL as an example:

select concat_ws(null, 'a', 'b', null, null, 'c');

• MSSQL: never returns null and treats null as empty string, so the above SQL results in "abc" (For more details, see mssql-concat_ws)

• MySQL and Postgresql: returns null when the separator is null, so the example SQL returns NULL.