Add skeleton loading for catalog list

What does this MR do and why?

Instead of the default spinner icon when loading ci/cd catalog resources, we want to implement a custom skeleton loader that will make the perceived performance faster and also general ease on the eyes.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make sure you have an ultimate license for your GDK
2. Enable the feature flag: ci_namespace_catalog_experimental
3. Login as a user
4. Create a few projects that you will be able to convert to Ci resources (ideally 20+).
5. Once you have done so, get the ID of the first new project you wanted to convert. Then in Rails console, run:

projects = Project.where("id > ?", your_first_project_id)
projects.each do |project|
  project.update!(description: 'description')  
  ::Ci::Catalog::Resource.new(project_id: project.id).save   
end  

1. Navigate to /$namespace/$project/-/ci/catalog/resources
2. Notice the loading state is now a skeleton loader and not a spinner

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #406812 (closed)