Simpler serialization for super sidebar

What does this MR do and why?

Simpler serialization for super sidebar

With !111186 (merged) we introduced serialization methods for Panels and Menus, in order to be able to use the existing Panels in the Navigation alpha.

In order to future-proof the concept, I decided to make those flexible, by flattening the tree-structure of Menus and Menu Items and then rebuilding it. The idea was to just provide different a different parent_id to certain menu items and they would be zipped up differently.

That being said, it is a little complicated and extending it to support different groupings and different kind of menus (e.g. the static menu at the top) proved difficult with this pattern. In a follow-up commit we will introduce a new pattern which extends the existing Panels and transforms them.

Contributes to #391967 (closed).

Screenshots or screen recordings

N/A (Same as !111186 (merged))

How to set up and validate locally

1. Enable the super_sidebar_nav and recommended: your_work_sidebar feature flags.
2. Log into the GDK and enable the New Navigation for the current user via the Profile Menu

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.