Add required option for nuxeo-directory-checkbox / nuxeo-directory-radio-button

nuxeo-directory-checkbox / nuxeo-directory-radio-button don't have a required option
=> If the field is required, it can't be enforced in the UI

Expected:

• Elements expose a required property
• If required property is activated
  • single valued field: one option has to be selected
  • multivalued field: at least one option has to be selected (doesn't really make sense to use the radio button kind in this case though since it's meant for single value selection)