sql: fix result type of min() and max() functions

Prior to this, the patch functions MIN(), MAX(), LEAST() and GREATEST()
showed SCALAR as result types in metadata. However, in reality, the type
of the result could be any scalar type. After this patch, the type of
the result will always be the same as the type in the metadata. Also,
for MIN() and MAX() functions, the type of the result will be the same
as the type of the argument. For the LEAST() and GREATEST() functions,
the result type will be the same as the type of the arguments if all
arguments are of the same type, or it will be NUMBER if all arguments
are of numeric types, or it will be SCALAR.

Part of #6105