Expose the conversion time of a "normal" skeleton to a stray which is in effect when a "normal" skeleton freezes due to powdered snow.

Instead of implementing it myself, I am opening this issue because of the following problem that needs to be addressed prior to an implementation of this feature and I wanted this issue to be a platform for discussion for it.

As of right now (1dff62ae830) the spigot API defines the Skeleton interface as a type of super class to other existing skeletons as well as the specific "normal" overworld skeleton type.
This layout proves somewhat difficult, as already shown through the countless deprecations in the Skeleton interface on methods like getSkeletonType as skeletons have different abilities/data assigned to them. Prior to the 1.17 release this was still a valid way to define a Skeleton as both the Stray and the Wither Skeleton shared all the features the "normal" skeleton had. The conversion to a Stray however is only present on the "normal" Skeleton and therefore cannot be present on either the stay or the wither skeleton.

Any potential suggestions to fix this issue in the API would be appreciated by me to come to a consensus in the community prior to me opening a PR with a potentially bigger change in the Skeleton API.