feat(api/entity): add EntityEvent.Push when an entity is pushed

[danorris709]

This PR is a continuation of the conversation from #9675.

Reason

The main reason I initially wanted to be able to modify the entity's push reaction was to prevent players from being pushed when in a specific GUI. In lower versions this was achievable by modifying the "pushthrough" variable on the Entity however, this was later removed by Mojang in favour of the "isPushable" method. In changing it to use this method they unfortunately removed the capability to prevent entities from being pushed outside of extending the entity and using your own class.

Details

Originally I added an event called when the isPushable method was called. However, when discussing the useful-ness I realized that perhaps adding functionality for modifying the push reaction would be more widely useful. There is only one other instance that this event doesn't cover which is the ItemFrame's push implementation as it eventually calls super anyway

From ItemFrame.java:

   public void push(double p_31817_, double p_31818_, double p_31819_) {
      if (!this.fixed) {
         super.push(p_31817_, p_31818_, p_31819_);
      }

   }

The only other consideration I had when making this PR was that potentially someone would want the "cause" of the push. There is a method LivingEntity#doPush however, it is a protected method and only defers to Entity#push. TL;DR: Currently I don't think it is pheasible to include the cause without creating a lot of changes in the Mojang code (which I assume is undesirable)

[danorris709]

Here is a GIF of the cow and player interactions from the test mod

[autoforge]

@danorris709, this pull request has conflicts, please resolve them for this PR to move forward.

[autoforge]

@danorris709, this pull request has conflicts, please resolve them for this PR to move forward.