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.

ci_scripts/ci_post_clone.sh

@@ -8,6 +8,10 @@ install_xcode_cloud_brew_dependencies
 
 if [ "$CI_WORKFLOW" = "Nightly" ]; then
     bundle exec fastlane config_nightly build_number:"$CI_BUILD_NUMBER"
+elif [ "$CI_WORKFLOW" = "Enterprise" ]; then
+    # Not sure what Xcode Cloud does, might need to also
+    # git submodule update --init --recursive
+    bundle exec fastlane config_enterprise
 else
     bundle exec fastlane config_production
 fi