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.

app.yml

@@ -0,0 +1,7 @@
+settings:
+  APP_DISPLAY_NAME: Element X
+  PRODUCTION_APP_NAME: Element
+  BASE_APP_GROUP_IDENTIFIER: io.element
+  BASE_BUNDLE_IDENTIFIER: io.element.elementx
+  ASSETCATALOG_COMPILER_GLOBAL_ACCENT_COLOR_NAME: "colors/accent-color"
+  DEVELOPMENT_TEAM: 7J4U792NQT