DatePicker and DateRangePicker assign date-only to FormControl

[zehavibarak]

Feature Description

Add getAssignValue(value: D) : any to DateAdapter.

This will enable returning a value other than date object. For example, string in YYYY-MM-DD format.

Use Case

Commonly, user selects a date with the intent of a date-only value. Date picker (and date range picker) returns date object in local time zone, which may reflet a date portion other than the one selected (adding or subtracting 1 day).

[mmalerba]

NativeDateAdapter works with native Date objects, but you could make a DateAdapter that works with any type you want. If you want a representation that doesn't include the time, I would recommend just building a DateAdapter that works that way. I'm not sure what the proposed getAssignValue function would do here, but I don't think we'd want to add something like this that just ignores all of the types.