Introduce Ci::Catalog::Resource and Listing models

What does this MR do and why?

Related to Create Ci::Catalog::Resource model (#393319 - closed)

In this MR we introduce Ci::Catalog:: namespace and a 2 models:

• Ci::Catalog::Resource which maps to the newly added catalog_resources table and indicates that a project of components is marked as being a catalog resource.
• Ci::Catalog::List is a higher-level model that taking in input a Namespace object it lists all catalog resources within the namespace's scope. Eventually this class could be expanded to support other ways of filtering catalog items (e.g. as a finder object), check restrictions, etc. Rather than using Project directly we should be using this class as SSoT for what's supported in the Catalog.

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