Rethink library architecture

[keeshux]

Reorganize the library into modules:

• Core
  • Common domain
  • Reusable components
  • Utils
• Providers
  • Domain of provider infrastructures
  • ProviderManager
• VPN
  • Domain of app profiles
  • ProfileManager
  • VPNManager

For each module, split entities by responsibility:

• Domain
  • Public: value objects
  • Managers: concrete ObservableObject for SwiftUI (mocking logic has no use)
  • Strategies: abstract and mockable
• Implementation
  • Private: mostly, or may instantiate via factory
  • Strategies: concrete implementations
  • Extensions: for convenience
  • 3rd parties: including Core Data

The refactoring enables/improves:

• SwiftUI Previews
• Testability
• Scalability
• Swapping dependencies