feat(userCache): add uuids() method

Author: imDMK

playtime-core/src/main/java/com/github/imdmk/playtime/core/feature/playtime/PlayTimeUserCache.java

@@ -40,6 +40,11 @@ Collection<PlayTimeUser> values() {
         return Collections.unmodifiableCollection(usersByUuid.values());
     }
 
+    @Unmodifiable
+    Collection<UUID> uuids() {
+        return Collections.unmodifiableCollection(usersByUuid.keySet());
+    }
+
     @Unmodifiable
     Collection<String> names() {
         return Collections.unmodifiableCollection(uuidByName.keySet());