Allow overriding built-in Admin UI nav menu items

[michaelbromley]

Is your feature request related to a problem? Please describe.
Currently it is possible to add menu items using the addNavMenuSection and addNavMenuItem function exposed by @vendure/admin-ui/core, but it is not possible to remove or override existing menu items. Internally the infrastructure is already in place to allow outside configuration of the entire nav menu. It just needs to be exposed.

Describe the solution you'd like
Two options:

1. Have addNavMenuSection and addNavMenuItem override any existing section or items if the id string matches an existing one.
2. Add new functions specifically for overriding.

But then how to remove menu items altogether? I guess that could be done by setting the requiresPermission property to some known invalid permission string. Then it would always be hidden in the admin UI.

Describe alternatives you've considered
I considered exposing a defineNavMenuSections(config: NavMenuSection[]) function which allows one to define the entire nav menu. The downside is that if you only want to override one or a few items, you end up having to duplicate the entire rest of the menu, which would be very inconvenient and verbose.