Subscription refactoring #5 (#874)

Task/Issue URL: https://app.asana.com/0/1205842942115003/1206805455884775/f
iOS PR: duckduckgo/iOS#3023
macOS PR: duckduckgo/macos-browser#2930
What kind of version bump will this require?: Major
Tech Design URL: https://app.asana.com/0/1205842942115003/1207147511614062/f

Dependency injection for many Subscription classes has been improved to allow unit tests.
Many test classes have been created and one or more example tests have been implemented to showcase the class testability and mocks
Complete tests will be implemented in follow-up tasks

.swiftpm/xcode/package.xcworkspace/xcshareddata/WorkspaceSettings.xcsettings

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>IDEWorkspaceSharedSettings_AutocreateContextsIfNeeded</key>
+	<true/>
+</dict>
+</plist>