Allow limiting access to menu items per-role

I'm making this request as part of the #1839 meta...

As a site builder, one can use the visibility conditions for the menu blocks they place on their layouts. If there is need to control access for specific menu items though, one either needs to code something custom, or use any of the following modules (none of them currently ported to Backdrop):

https://www.drupal.org/project/menu_item_role_access (D8-only - usage: 4k+ sites) https://www.drupal.org/project/menu_item_visibility (D7-only / in beta since 2017 / 24k+ installations) https://www.drupal.org/project/menu_per_role (both D7/D8 / 14k installations) https://www.drupal.org/project/roles_for_menu (D7-only / 5k installations)

The combined usage of these modules (that basically do the same thing) is 40k+ - not a very compelling reason if one considers that there are 1mil+ Drupal sites out there, but a considerable usage nonetheless.

Before we port one of them to Backdrop, I would like us to explore the possibility of adding this feature in core. If the code changes required to get this in are considerable, then we should leave it to contrib.

Some screenshots of the UI of these contrib modules: