Add keyStream() API to registries (#12479)

paper-server/src/main/java/io/papermc/paper/registry/legacy/DelayedRegistry.java

@@ -49,6 +49,11 @@ public final class DelayedRegistry<T extends Keyed, R extends Registry<T>> imple
         return this.delegate().stream();
     }
 
+    @Override
+    public Stream<NamespacedKey> keyStream() {
+        return this.delegate().keyStream();
+    }
+
     @Override
     public int size() {
         return this.delegate().size();