fix(b-dropdown-item/b-dropdown-item-button): Allow to configure `role` attribute for dropdown item

This change will allow to provide role and listItemRole properties for BDropdownItem and BDropdownItemButton. This will allow to configure role both for li item as well as a/button element to allow different dropdown types to meet accessibility criteria. For instance, disclosure should not have role set where menu should have menuitem set for the list item's a/button.