loot tracker: add metadata field to loot record

This is for tracking miscellaneous data with the loot records, such as
npc id and skill levels, for the wiki drop log project.

Co-authored-by: Adam <Adam@sigterm.info>

http-api/src/main/java/net/runelite/http/api/loottracker/LootRecord.java

@@ -37,6 +37,7 @@ public class LootRecord
 {
 	private String eventId;
 	private LootRecordType type;
+	private Object metadata;
 	private Collection<GameItem> drops;
 	private Instant time;
 }