Explore new hook-based extension mechanism for the primary navigation

[stopfstedt]

Currently, we're extending the primary nav class in order to drive course-category-specific items into the primary navigation.
With M4.3, there is now a new mechanism for extending the primary nav that does not require sub-classing.

https://tracker.moodle.org/browse/MDL-79088

This may even allow us to extract this feature into a standalone plugin.

Also check out the reference implementation https://github.com/marinaglancy/moodle-tool_sitenavdemo

[stopfstedt]

couple of notes:

• rigging up the hook is really easy to accomplish
• the custom_menu API that we're currently using to push our category-based additions into the primary nav will not work here. it is incompatible with the interface that core\navigation\views\primary is providing.
• we'd therefore have to provide our own input parser, which could be modeled on custom_menu::convert_text_to_menu_nodes()