Make the bone-to-pet-wolf transformation item customizable.

A new section in global-settings called pet-items is used to define pet item transformation items. By default, bones are transformed into wolves, but the item type can now be changed to support using actual bones in classes.

This refactors the pet spawning into its own class to reduce some of the massive responsibility of ArenaImpl.

This closes #467

src/main/resources/res/global-settings.yml

@@ -1,4 +1,6 @@
 enabled: true
 allowed-commands: /list, /pl
 update-notification: true
-prefix: '&a[MobArena] '
+prefix: '&a[MobArena] '
+pet-items:
+  wolf: bone