Admin Sidebar: add (optional) icon support to extendable submenu items #6117

Is your feature request related to a problem? Please describe.

Currently, expandable submenu items in the admin sidebar only display text labels

Some institutions define specific styles for submenu icon and label pairs as part of their visual identity system

Describe the solution you'd like

Introduce an optional icon property to extendable submenu models

Describe alternatives or workarounds you've considered

Currently, adding icons to these submenu items requires editing core templates and/or creating custom theme overrides, which complicates future DSpace upgrades

Additional information

#6115 has been prepared to implement this update