[Feature Request]: Calculator - add Date generator

### What would you like to happen?

In the Calculator transform there are many operations to get and modify the various parts of a Date (year, month, day of month, day of week, etc.), ma no operation to get a Date object from its parts (year, month, day of month).

It could be useful to add an operation like "Create Date from Year A, Month B and Day of month C": the time part can be defaulted to midnight (and then modified with the already existing operations "Date A + B hours/minutes/seconds").

The current workaround consists in using a User Defined Java Expression transform, and create a field of type `Date` with an expression like:
`new java.util.GregorianCalendar(YEAR_FIELD.intValue(), MONTH_FIELD.intValue() - 1, DAY_OF_MONTH_FIELD.intValue()).getTime()`
_(beware of the -1 in the month field!)_

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui, Component: Transforms

Start at the Calculator transform and review its existing date-part operations and the User Defined Java Expression workaround described in the issue. Add an operation that creates a Date from year, month, and day-of-month fields with the time set to midnight, then verify that the resulting Date works with the existing date adjustment operations.

Written by the indexing model from the issue text.