Checkbox/Radio enable them to be required

**Is your feature request related to a problem? Please describe.**
Unable to require a group of radio or checkboxes to be selected on a form

**Describe the solution you'd like**
Support `optional` prop for `RadioGroup` and `CheckboxGroup`

Start by locating the RadioGroup and CheckboxGroup component entry points and read how their current props and selection behavior are tested. Add support for the requested optional prop to both groups, then verify that required and optional selection behavior is covered by the relevant component tests.

Written by the indexing model from the issue text.