Add SolidusAdmin

[elia]

Add SolidusAdmin to the list solidus sub-frameworks, for now it will live in the repo and be released separately while we complete coverage for the existing backend sections.

During the intermediate phase it will be possible to keep SolidusAdmin and Spree::Backend in the same rails application and benefit immediately from the updated UI/UX of the sections covered by the new admin dashboard.

Features

• ✨ Updated modern UI, more intuitive and familiar to modern users
• 🧰 A dedicated UI Kit that can be used to easily create custom sections with high quality and low effort
• 🌱 A built-in switch that allows to fall back on the legacy admin dashboard that allows a soft transition and early feedback
• 🎛️ A components registry that will enable controlled updates to the UI/UX and to completely abandon UI monkey patching (e.g. with deface)

Action Items:

• Consider creating an hosted demo for the new admin interface before the upcoming release. This would provide an interactive way to showcase the improvements and new features.

[codecov]

Codecov Report

Merging #5068 (1288f9c) into main (803be9d) will increase coverage by 0.02%.
The diff coverage is 89.30%.

❗ Current head 1288f9c differs from pull request most recent head a5372d3. Consider uploading reports for the commit a5372d3 to get more accurate results

@@            Coverage Diff             @@
##             main    #5068      +/-   ##
==========================================
+ Coverage   88.74%   88.77%   +0.02%     
==========================================
  Files         565      609      +44     
  Lines       13986    14762     +776     
==========================================
+ Hits        12412    13105     +693     
- Misses       1574     1657      +83     

Files	Coverage Δ
...app/components/solidus_admin/feedback/component.rb	100.00% <100.00%> (ø)
...ponents/solidus_admin/products/status/component.rb	100.00% <100.00%> (ø)
...nts/solidus_admin/sidebar/account_nav/component.rb	100.00% <100.00%> (ø)
.../app/components/solidus_admin/sidebar/component.rb	100.00% <100.00%> (ø)
...components/solidus_admin/sidebar/item/component.rb	100.00% <100.00%> (ø)
...pp/components/solidus_admin/skip_link/component.rb	100.00% <100.00%> (ø)
...app/components/solidus_admin/ui/badge/component.rb	100.00% <100.00%> (ø)
...pp/components/solidus_admin/ui/button/component.rb	100.00% <100.00%> (ø)
...nents/solidus_admin/ui/forms/checkbox/component.rb	100.00% <100.00%> (ø)
...mponents/solidus_admin/ui/forms/field/component.rb	100.00% <100.00%> (ø)
... and 35 more

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[rainerdema]

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