Add utility function to ItemStack for creating custom PlayerHeads

[NotThatRqd]

Objective

• Make it easier to create PlayerHeads with a custom skin.

Solution

• Quick utility function on the ItemStack struct that will add the necessary NBT for you.

Concerns

• Not sure if my explanation of how you're supposed to use the function (in the rustdoc) is good enough.
• Not sure if returning a Result on a function meant to be chained is okay.
• Not sure if I've chosen a good name for the function.