WPB-24805 [iOS] improve FilesViewModel of WireDrive

Issue

The swift file FilesViewModel is almost 1k loc long, is shared by multiple Views and has accumulated a lot of code for different kinds of functionality.
This makes it hard to read and maintain it.

Solution

Many different kinds of refactoring were implemented resulting in the FilesViewModel.swift file being about 50% smaller:

• extracted FilesViewItem into a separate file
• refactored SheetNavigation to use Model data instead of SwiftUI Views as payload, eliminating the need for import SwiftUI in ViewModel code
• refactored sub-views and sheets to consistently use callbacks instead of Combine bindings to notify the parent view about specific events
• unified creation of sub-views or sheets to only create the ViewModels rather than SwiftUI Views
• extracted all functions which create ViewModels into the extension FilesViewModel+make_ViewModels.swift
• extracted the big UseCases nested struct into the extension FilesViewModel+UseCases.swift
• extracted the big function for loading offline files into LoadOfflineAvailableFilesUIAction
• extracted logic related to loading paginated lists of (file) items into the components IncrementalListLoader and FilesListLoader
• moved and grouped all of the remaining functions using // MARK: label comments

Testing

The logic should be exactly the same after the refactoring.

• Go to the files list, via the Drive tab or the Shared Drive of a conversation
• Check if the list is loaded correctly and the pagination is right
• Check offline mode and if the list of offline available files is loaded correctly
• Check deleting and restoring files via recycle bin