related: #98545

We want to add an mv_union function which should receive a variable number of arguments.
We should support all data types that can be multi valued.

Some examples:

ROW a = MV_UNION([1,2,3], [2,3,4], [1,2]) -> [1,2,3,4]
ROW a = MV_UNION([1,2,3], 1) -> [1,2,3,4]
ROW a = MV_UNION(1, 2) -> [1,2]
ROW a = MV_UNION(1) -> 1