[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

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
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• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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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)

[FirentisTFW]

Already merged in parent repository, this one served just as a preview. Closing