Add HiddenPotionEffect API

== AT ==
public net.minecraft.world.effect.MobEffectInstance hiddenEffect

paper-server/src/main/java/org/bukkit/craftbukkit/potion/CraftPotionUtil.java

@@ -78,6 +78,7 @@ public class CraftPotionUtil {
 
     public static MobEffectInstance fromBukkit(PotionEffect effect) {
         Holder<MobEffect> type = CraftPotionEffectType.bukkitToMinecraftHolder(effect.getType());
+        // Paper - Note: do not copy over the hidden effect, as this method is only used for applying to entities which we do not want to convert over.
         return new MobEffectInstance(type, effect.getDuration(), effect.getAmplifier(), effect.isAmbient(), effect.hasParticles(), effect.hasIcon()); // Paper
     }
 
@@ -87,7 +88,7 @@ public class CraftPotionUtil {
         int duration = effect.getDuration();
         boolean ambient = effect.isAmbient();
         boolean particles = effect.isVisible();
-        return new PotionEffect(type, duration, amp, ambient, particles, effect.showIcon()); // Paper
+        return new PotionEffect(type, duration, amp, ambient, particles, effect.showIcon(), effect.hiddenEffect == null ? null : toBukkit(effect.hiddenEffect)); // Paper
     }
 
     public static boolean equals(Holder<MobEffect> mobEffect, PotionEffectType type) {