[AVFoundation] APIs that take AVFileTypes are bound as string without type safe helper variants

[tipa]

I am in the process of migrating my apps from legacy Xamarin to .NET6.
Previously, I could use AVFileType.Mpeg4 to set the property OutputFileType of AVAssetExportSession.
Now the AVFileType type has gone and I have to use AVFileTypes.Mpeg4.GetConstant() instead (correct?).

Wouldn't it make sense to then also change the OutputFileType (and SupportedFileTypes) property types of AVAssetExportSession to AVFileType? and AVFileType[] respectively?

This would then also better match the types documented by Apple: https://developer.apple.com/documentation/avfoundation/avassetexportsession/1387110-outputfiletype
For developers it is not intuitive to use a method on an enum to get the string required.

Related issue:
#8735
#15683

[chamons]

I can confirm, we have those bound as string OutputFileType today. In other cases with smart enums we so something like:

		[Export ("type")]
		NSString WeakType { get;  }

		public AVMetadataObjectType Type {
			get {
				return AVMetadataObjectTypeExtensions.GetValue (WeakType);
			}

So this is one area we can improve the bindings, though getting a name for the type safe version will be tricky.