Goal

Contribute Custom menu items in the New Frontend Application navigation bar.

Background/Feature Origin

Currently the upstream New Frontend Application Navigation Bar extension doesn't support custom navigation menu items (whose react code is provided as a plugin). It only supports menu items with a fixed icon, name and target route.

We should contribute a first simple implementation.
It might then trigger review cycles upstream and require refining the approach which would be tackled by another issue. 

Why is this important?

This is a requirement to have feature parity for the Notifications plugin for example.
OTOH, it's an easy contribution to bring upstream.