#18547 introduces CGImage<->Mat conversion for MacOS platforms. This PR enables the same capabilities for iOS platforms. C++ users can now use MatToCGImage and CGImageToMat in ios.h, and Objc users can now do the equivalent conversions in Mat+Converters.h.

I also took the liberty of adding CF_RETURNS_RETAINED annotations to methods that return CGImageRef, so that the Swift compiler knows to CFRelease() when the reference count goes to 0. This is technically a breaking change, as the prior implementation required users to explicitly use takeRetainedValue() to properly reference count; however, I think the expectation is that such objects are reference counted by default, so I would categorize the prior behavior as a bug and my changes as a bugfix.

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders_only=Custom Mac
build_image:Custom Mac=osx_framework-test
buildworker:Custom Mac=macosx-2
build_contrib:Custom Mac=OFF