Support autoreverses in gradient animations

[Juanpe]

Fixes #278

Summary

Now, for gradient skeletons, we can decide if the animation has autoreverses property enabled or not. If this property is true
the animation will be played with a ping-pong effect.

Preview:

How to use:
When launching the skeleton, you need to specify this property, here an example:

view.showAnimatedGradientSkeleton(animation: GradientDirection.leftRight.slidingAnimation(autoreverses: true))

Requirements

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.