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/Hooks/AppHooks.swift

@@ -0,0 +1,45 @@
+//
+// Copyright 2024 New Vector Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+import Foundation
+
+// MARK: Registration
+
+class AppHooks: AppHooksProtocol {
+    private(set) var appSettingsHook: AppSettingsHookProtocol?
+    func registerAppSettingsHook(_ hook: AppSettingsHookProtocol) {
+        appSettingsHook = hook
+    }
+    
+    func runAppSettingsHook(_ appSettings: AppSettings) -> AppSettings {
+        guard let appSettingsHook else { return appSettings }
+        return appSettingsHook.run(appSettings: appSettings)
+    }
+}
+
+protocol AppHooksProtocol {
+    func configure()
+}
+
+extension AppHooksProtocol {
+    func configure() { }
+}
+
+// MARK: Protocols
+
+protocol AppSettingsHookProtocol {
+    func run(appSettings: AppSettings) -> AppSettings
+}