[video_player] Convert top-level classes to Swift (#11658)

Converts the top-level plugin class, along with the native view factory that is used for platform view management, from Objective-C to Swift. The Pigeon interface for the plugin class has been split out into a separate definition file so that it can use the Swift Pigeon generator while the player-instance-level API can continue to use Obj-C for now.

Since this is the first use of Swift in the plugin, this includes separating the Swift Package into more modules; now instead of:
video_player_avfoundation (Obj-C) -> video_player_avfoundation_ios and _macos (both Obj-C)
it's
video_player_avfoundation (Swift) -> video_player_avfoundation_objc (Obj-C) -> video_player_avfoundation_ios and _macos (both Obj-C)

The Swift conversions were done via Gemini as a starting point, but I did a side-by-side comparison to the Obj-C to ensure that everything was actually preserved, and fixed issues in its conversion manually.

The PR also includes adding some NONNULL region annotations to Obj-C headers that were accidentally missing it, since I ran into one of the omissions once I was calling the API from Swift, and then I audited all of the headers.

Part of https://github.com/flutter/flutter/issues/119105

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