Expand on entity serialization API (#11807)

paper-api/src/main/java/io/papermc/paper/entity/EntitySerializationFlag.java

@@ -0,0 +1,38 @@
+package io.papermc.paper.entity;
+
+import org.bukkit.UnsafeValues;
+import org.bukkit.entity.Entity;
+import org.bukkit.entity.Player;
+
+/**
+ * Represents flags for entity serialization.
+ *
+ * @see UnsafeValues#serializeEntity(Entity, EntitySerializationFlag... serializationFlags)
+ * @since 1.21.4
+ */
+public enum EntitySerializationFlag {
+
+    /**
+     * Serialize entities that wouldn't be serialized normally
+     * (e.g. dead, despawned, non-persistent, etc.).
+     *
+     * @see Entity#isValid()
+     * @see Entity#isPersistent()
+     */
+    FORCE,
+    /**
+     * Serialize misc non-saveable entities like lighting bolts, fishing bobbers, etc.
+     * <br>Note: players require a separate flag: {@link #PLAYER}.
+     */
+    MISC,
+    /**
+     * Include passengers in the serialized data.
+     */
+    PASSENGERS,
+    /**
+     * Allow serializing {@link Player}s.
+     * <p>Note: deserializing player data will always fail.
+     */
+    PLAYER
+
+}