Use XcodeGen files for app variants and setup Enterprise submodule. (#2976)

- Create an app.yml with the base app variant.
- Override app.yml contents on Nightly/PR builds.
- Add AppHooks mechanism.
- Add Enterprise submodule.
- Conditionally import Enterprise/fastlane/Fastfile if available.

ElementX/Sources/Mocks/PHGPostHogMock.swift

@@ -31,7 +31,7 @@ class MockPostHogFactory: PostHogFactory {
         self.mock = mock
     }
     
-    func createPostHog(config: PostHogConfig) -> ElementX.PHGPostHogProtocol {
+    func createPostHog(config: PostHogConfig) -> PHGPostHogProtocol {
         mock
     }
 }