Allow URL argument in setSkullTexture()

[EvModder]

For the setSkullTexture() method in SkullUtils.java:

public static void setSkullTexture(@NonNull ItemStack itemStack, @NonNull String texture);

... Allow the texture argument to be either a Base64 or URL, with some basic validation

[EvModder]

Unfortunately I don't have the development branch in which I originally drafted this PR, and although I could recreate it I also just don't have as many free cycles in my day for open source community contributions like this at the moment :(

It looks like it's pretty close to complete how the project owners want it to be, so maybe another author can step in and complete this.

For context, I only sent this PR because someone from the DecentHolograms dev team was asking about it in a mutual Discord, I am not actually a regular contributor (or even a user) of this project, so I just wanted to give an example of how they could support it.