[video_player_avfoundation] 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 FVPVideoPlayerTextureApproach - 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 method, viewType parameter in DataSource).

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 will be renamed to just playerId. This will be done in a separate PR to keep git diff for this one clean.

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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[FirentisTFW]

I'm not sure if this is the best approach, but looks like texture ids start from 0 and then get incremented, so this starting point seems to be pretty safe. Alternatively, I assume we could generate random ids for non-texture players

[FirentisTFW]

One PR check is failing, but this is due to changes in platform interface and local dependency overrides. The code can be reviewed already. Once this PR is reviewed, I'll create another one with platform interface changes only (according to contributing/README.md).