Add leash events

[haykam821]

This pull request adds events for attaching and detaching entities from leashes (using the vanilla lead item, for example). In particular, the following two events are added:

• Attach
  • Interacting with an unleashed entity while holding a lead
  • Interacting with a leash knot while leashing an entity
  • Interacting with a fence containing a leash knot while leashing an entity
• Detach
  • Interacting with an entity leashed by the player

Entity deserialization-related attachment and detachment is not included in these events. I am not sure whether or how causes such as a leash knot breaking and detaching entities due to not being supported by a fence or a leashed entity being too far away from its leash holder should be considered in these events.