Add entity knockback events

- EntityKnockbackEvent
- EntityPushedByEntityAttackEvent
- EntityKnockbackByEntityEvent

Co-authored-by: aerulion <aerulion@gmail.com>
Co-authored-by: Jake Potrebic <jake.m.potrebic@gmail.com>

paper-api/src/main/java/org/bukkit/event/entity/EntityKnockbackByEntityEvent.java

@@ -7,7 +7,10 @@ import org.jetbrains.annotations.NotNull;
 
 /**
  * Called when an entity receives knockback from another entity.
+ *
+ * @deprecated use {@link com.destroystokyo.paper.event.entity.EntityKnockbackByEntityEvent}
  */
+@Deprecated(forRemoval = true) // Paper
 public class EntityKnockbackByEntityEvent extends EntityKnockbackEvent {
 
     private final Entity source;

paper-api/src/main/java/org/bukkit/event/entity/EntityKnockbackEvent.java

@@ -11,7 +11,10 @@ import org.jetbrains.annotations.NotNull;
 
 /**
  * Called when a living entity receives knockback.
+ *
+ * @deprecated use {@link io.papermc.paper.event.entity.EntityKnockbackEvent}
  */
+@Deprecated(forRemoval = true) // Paper
 public class EntityKnockbackEvent extends EntityEvent implements Cancellable {
 
     private static final HandlerList handlers = new HandlerList();