Migrate iOS embedder to use the embedder API

The current planned steps are: (Check marked items are in flutter/main)

• Setup a runtime flag to enable and disable Embedder api during the migration.
• ☂️ Migrate iOS embedder objective-c code to ARC (Auto Reference Counting) #137801
• Initialize the FlutterEngineProcTable& embedderAPI property, with callbacks left as TODOs.
• Implementing features. This can be separated to smaller patches, enable tests along the way if possible
• Run all the tests with the embedder API, make sure there are no meaningless regressions.
• Remove legacy code and make embedder API stable.

In step 3 above, some features are non-trivial. They are listed are listed below: (Check marked items are in flutter/main)

• Initialize FlutterEngine with EmbedderAPI.
• Render content on FlutterView.
• Background Platform Channel message
• Touch events.
• Keyboard Events.
• External View Embedder (PlatformView)
• SemanticsUpdate
• VsyncWaiter

Additional graphics-related work

• Impeller integration
• Swapchain integration
• Wide gamut
• Hook up graphics debugger

A draft PR that contains the full migration is maintained here: flutter/engine#39988 as a reference while working on individual patches.

Below is a list of completed features in the draft PR. (Check marked items are the ones completed in the flutter/engine#39988)

• Initialize FlutterEngine with EmbedderAPI.
• Render content on FlutterView.
• Background Platform Channel message. (Workaround)
• Background Platform Channel message.
• Touch events.
• Keyboard Events.
• External View Embedder (PlatformView) (In progress)
• SemanticsUpdate
• VsyncWaiter

Currently, the embedder API doesn't support impeller. #112230 is required to enable iOS to use impeller by default after the migration.