add flatten method on object codecs

Author: mudkipdev

json/src/main/java/alpine/json/codec/Codec.java

@@ -142,6 +142,10 @@ public <R> R encode(Transcoder<R> transcoder, E value) {
         };
     }
 
+    default FlatCodec<T> flatten() {
+        return new FlatCodec<>(this);
+    }
+
     static <T> CodecBuilder._0<T> builder() {
         return new CodecBuilder._0<>();
     }