Add animationDidFinish API to LottieView

[calda]

This PR adds an animationDidFinish API to LottieView.

When using loopMode: .playOnce, or another finite duration animation like .repeat(3), this lets you know when the animation is complete.

For example, in the example app you can see we previously didn't update the "pause" button to the "play" state when the animation ended. Now we do:

.animationDidFinish { completed in
  if completed {
    animationPlaying = false
  }
}

Before	After

[miguel-jimenez-0529]

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