Make #date_increment configurable in DateRangeDefaultWidget and DateTimeDefaultWidget

Problem/Motivation

The datetime element supports a #date_increment property that allows limiting the precision of date values
This cannot be configured with the default widgets that ship with core

Proposed resolution

Make DateRangeDefaultWidget and DateTimeDefaultWidget support configuring the #date_increment property

Remaining tasks

Everything

User interface changes

API changes

Data model changes

Release notes snippet