[engine] Sync Flutter 3.7 source code

[swift-kim]

Copy common headers and source code (including embedder.h and cpp_client_wrapper) from the Flutter 3.7 engine source tree except accessibility-related code.

TODO:

• Fix the black screen and flickering issue.
  • There will be a follow-up PR soon.
• Implement a thread-safe messenger (FlutterDesktopMessengerAddRef and etc.)
  • Refer to https://github.com/flutter/engine/pull/36909/files.
  • We don't currently have a plan to implement this. Let's revisit when it becomes necessary.
• Sync the accessibility-related code (including third_party/accessibility).
• Replace deprecated update_semantics_node_callback with update_semantics_callback.
  • Refer to https://github.com/flutter/engine/pull/37404/files.
• Migrate plugins to use TextureRegistrar::UnregisterTexture(texture_id, callback) instead of deprecated TextureRegistrar::UnregisterTexture(texture_id).

[swift-kim]

The TODO items will be implemented by separate PRs. This PR is now ready for review.