SolidusAdmin: Extract page layout helpers

[elia]

Summary

Extract some helpers that can be used to build page layouts, this is the first step in sharing this code, we're looking toward a better solution for the future (e.g. one or multiple components, or prebuilt TW classes) but it's early to tell which will be the best approach.

Checklist

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• I have written a thorough PR description.
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[codecov]

Codecov Report

Merging #5445 (510878d) into main (de6030b) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #5445      +/-   ##
==========================================
+ Coverage   88.89%   88.91%   +0.02%     
==========================================
  Files         612      614       +2     
  Lines       14800    14827      +27     
==========================================
+ Hits        13156    13183      +27     
  Misses       1644     1644              

Files	Coverage Δ
...pp/components/solidus_admin/layout/page_helpers.rb	100.00% <100.00%> (ø)
...p/components/solidus_admin/orders/new/component.rb	100.00% <100.00%> (ø)
...mponents/solidus_admin/products/index/component.rb	91.17% <100.00%> (+0.26%)	⬆️
...omponents/solidus_admin/products/show/component.rb	78.57% <100.00%> (+1.64%)	⬆️
...onents/solidus_admin/ui/table/toolbar/component.rb	100.00% <100.00%> (ø)

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