[ic-side-navigation] Drill down menus

## Summary
Following on from all the work on side nav, the remainder of the work that needs to be defined is:
- Drill down menus

## 💰 Use value
Drill down menus offer a multi-level tree structure for the user to select the metadata value from.

No file, entry point, or test is named. Start by reviewing the existing side-navigation work, then clarify the required multi-level tree behavior and acceptance criteria; the work is done when drill-down menu behavior is defined and implemented consistently with that navigation.

Written by the indexing model from the issue text.