[ios, macos] possibility to set custom images to style (#6637)
* [ios, macos] possibility to set custom images to style
* [ios, macos] setImage:forName now supports MGLImage (UIImage/NSImage)
* [ios, macos] update documentation for setImage:forName: and removeImageForName:
* [ios, macos] rename method and fix prefix
* [ios, macos] change header visibility to project
* [ios, macos] update header imports
* [ios, macos] delete unnecessary whitespaces
* [ios, macos] remove unnecessary nil checks
* [ios, macos] delete unnecessary whitespaces
* [ios, macos] update documentation
* [ios, macos] make mgl_spriteImage a parameter-less instance method
* [ios, macos] add asserts
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