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/com/destroystokyo/paper/event/entity/EntityKnockbackByEntityEvent.java

@@ -0,0 +1,52 @@
+package com.destroystokyo.paper.event.entity;
+
+import io.papermc.paper.event.entity.EntityKnockbackEvent;
+import io.papermc.paper.event.entity.EntityPushedByEntityAttackEvent;
+import org.bukkit.entity.Entity;
+import org.bukkit.entity.LivingEntity;
+import org.bukkit.util.Vector;
+import org.jetbrains.annotations.ApiStatus;
+import org.jspecify.annotations.NullMarked;
+
+/**
+ * Fired when an Entity is knocked back by the hit of another Entity. The acceleration
+ * vector can be modified. If this event is cancelled, the entity is not knocked back.
+ */
+@NullMarked
+public class EntityKnockbackByEntityEvent extends EntityPushedByEntityAttackEvent {
+
+    private final float knockbackStrength;
+
+    @ApiStatus.Internal
+    public EntityKnockbackByEntityEvent(final LivingEntity entity, final Entity hitBy, final EntityKnockbackEvent.Cause cause, final float knockbackStrength, final Vector knockback) {
+        super(entity, cause, hitBy, knockback);
+        this.knockbackStrength = knockbackStrength;
+    }
+
+    /**
+     * @return the entity which was knocked back
+     */
+    @Override
+    public LivingEntity getEntity() {
+        return (LivingEntity) super.getEntity();
+    }
+
+    /**
+     * @return the original knockback strength.
+     * @apiNote this value doesn't necessarily relate to {@link #getKnockback()}.
+     */
+    @ApiStatus.Obsolete(since = "1.20.6")
+    public float getKnockbackStrength() {
+        return this.knockbackStrength;
+    }
+
+    /**
+     * Gets the causing entity. Same as {@link #getPushedBy()}.
+     *
+     * @return the Entity which hit
+     */
+    public Entity getHitBy() {
+        return super.getPushedBy();
+    }
+
+}