[video_player_avfoundation] iOS platform view support

[FirentisTFW]

This PR adds support for platform views on iOS as a way of displaying a video. When creating a video, it's now possible to choose between texture view approach (rendered using Texture widget on the Flutter side) and platform view approach (rendered on the native side, using AVPlayerLayer).

FVPVideoPlayer class now has nothing to do with texture. The texture-related code was moved from it to FVPTextureBasedVideoPlayer - a subclass of FVPVideoPlayer that adds texture functionality. In the plugin class (createWithOptions method) we create either the basic version (for platform view) or the texture subclass (in case of texture approach) based on the parameter passed in from Flutter side.

Platform view is only supported on iOS, no MacOS implementation is added in this PR. The functionality is not yet exposed in the app-facing package (only in example app) - it will be done later, once we add the Android implementation. The PR does not introduce breaking changes, I followed the rule "non-breaking changes, even at the expense of a less-clean API" (buildViewWithOptions and createWithOptions methods).

Up to this point, the variable naming relied heavily on the texture (we had a lot of textureId variables and properties). Since now you can use a platform view instead of a texture view, these variables and parameters were renamed to just playerId.

I left some comments in the PR to clarify/discuss some choices.

Resolves #86613 (not fully though, as it is not yet available in the app-facing package).

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[stuartmorgan-g]

LGTM with minor comments (none that need another round of review, and feel free to ignore the optional ones if you disagree with them). For the Id/Identifier thing, you can use your judgement about where to stop renaming.

If there's anything you'd specifically like me to re-review let me know and I'll take a look, but otherwise feel free to land once the comments are addressed!

[FirentisTFW]

@stuartmorgan @hellohuanlin Thanks for the review, I addressed all of your comments.

For Id/Identifier - I renamed the Id to Identifier where possible. It worked great, now Id is mostly left in pigeon-generated files.
I also applied optional suggestions from the last review.

Thanks for the iteration here, I'm merging it now.