Describe the bug
Up until 0.24, components could define dynamic admin stylesheets to be included in the admin stylesheet as shown here:

After the webpacker migration, I this is no longer possible.

To Reproduce

1. Define an admin stylesheet for a component using component.admin_stylesheet = "...".
2. See if the stylesheet got included in the admin styles

Expected behavior
We should apply a similar fix for the admin stylesheet inclusions as we did for the dynamic front-end stylesheet inclusions at:
#8115

I suggest the following register method for the admin stylesheets:

Decidim::Webpacker.register_stylesheet_import("stylesheets/decidim/foobar/admin", group: :admin)

Screenshots
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Stacktrace
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Extra data (please complete the following information):

• Device: (any)
• Device OS: (any)
• Browser: (any)
• Decidim Version: 0.25.0+
• Decidim installation: (any)

Additional context
N/A