Backend: Prepare Ci::Catalog::Listing for global CI Catalog

Problem

The Ci::Catalog::Listing class currently takes in input a namespace object to scope resources to a given namespace view. Our goal is to move away from scoping resources to a namespace.

We also want to allow an easier way to add filters, possibly combining existing project filters (from the Project Explorer page) with new ones.

Proposal

Refactor Ci::Catalog::Listing to:

• remove namespace scope.
• make it work in scenarios where current_user is nil (not authenticated).
• Reuse ProjectsFinder internally if possible (allows to reuse existing filters).