[DRAFT] video_player 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 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.

Pre-launch Checklist

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[FirentisTFW]

Whole file content extracted from FVPVideoPlayerPlugin.m

[FirentisTFW]

This code was a part of FVPVideoPlayer class. It is all related to texture approach. I extracted it from FVPVideoPlayer and put here. This class extends FVPVideoPlayer and adds texture approach specific code.

[FirentisTFW]

I had to add many of these methods to the interface because they were referenced in FVPVideoPlayerPlugin.m (and now the player is not longer a part of that file)