Parent issue: #16092

iOS application extensions are extra functionality that run outside an app, when the app is not running.

This umbrella issue is to track the individual tasks to enable the support of app extension on iOS and macOS with approach 2 in the below design doc.

Design doc: https://flutter.dev/go/app-extensions

Task

• [app_extension_safe] engine build flag to expose an extension safe Flutter.framework/FlutterMacOS.framework #124286
• [app_extension_safe] Make iOS embedder extension safe when extension_safe build flag is true #124289
• [app_extension_safe] Make macOS embedder extension safe  #124290 (macOS)
• Cannot create a Flutter macOS app extension: 'FlutterEngineInitialize' returned 'kInvalidArguments'. The assets path in the Flutter project arguments was missing. #124755 (macOS)
• [app_extension_safe] Add integration tests for extension safe framework #124292
• [app_extension_safe] CI Build step to build and upload extension-safe Flutter.framework #124291
• [app_extension_safe] update documents for instructions on how to use app-extension-safe framework #124293
• Add new artifacts to codesigning script and any recipe changes (coordinate with @XilaiZhang and @christopherfujino) https://github.com/christopherfujino/codesign.py/blob/master/codesign.py
• Update conductor to validate codesigning the new artifacts, this may require a manual engine roll (coordinate with @XilaiZhang)

  flutter/dev/conductor/core/lib/src/codesign.dart

  Lines 194 to 199 in 73bd978

  	'artifacts/engine/ios-profile/Flutter.xcframework/ios-arm64/Flutter.framework/Flutter',
  	'artifacts/engine/ios-profile/Flutter.xcframework/ios-arm64_x86_64-simulator/Flutter.framework/Flutter',
  	'artifacts/engine/ios-release/Flutter.xcframework/ios-arm64/Flutter.framework/Flutter',
  	'artifacts/engine/ios-release/Flutter.xcframework/ios-arm64_x86_64-simulator/Flutter.framework/Flutter',
  	'artifacts/engine/ios/Flutter.xcframework/ios-arm64/Flutter.framework/Flutter',
  	'artifacts/engine/ios/Flutter.xcframework/ios-arm64_x86_64-simulator/Flutter.framework/Flutter',

• Update g3 scripts to roll new artifact into g3 and update build rules so 1p apps can use for app extensions (coordinate with @chingjun)
• Enable Scene Life Cycle Delegate for plugins #130445
• iOS app extension errors with "Cycle inside Runner" when "Thin Binary" Build Phase is before "Embed Foundation Extensions" #135056