[unity] Provide alternative to [SpineSkin] string with cached Skin reference

[HaraldCsaszar]

It's bad to request users to use [SpineSkin] string skin; and tell them to use FindSkin(skin) and cache the returned Skin reference themselves all the time.

Instead e.g. provide a class which holds both the cached Skin reference and a serialized string (or something similar).

Alternatively, a SerializedAsset analogous to AnimationReferenceAsset could be used. Benefit would be to only cache the skin reference once instead of at each instance.