Transform.day does not produce DateType value

[chenjunjiedada]

Transform.day was expected to produce date type, see: #279. It does produce the date type but not date value. Actually it produces the integer value. I think we should add a real date transform Transform<T, DateType>.

[rdblue]

Transform.day produces the internal value for a date, the number of days from the unix epoch. Iceberg uses that representation, not a Java date type.