[Java Edition] Allow function tags in Advancement rewards

Looking at https://minecraft.gamepedia.com/Advancements#JSON_Format, it does not show that the "function" field within the advancement reward JSON object support function tags.

After I tested in 19w12b, it indeed does not support such function tags.

It should be technically easy to add such a feature (as "function" command already support calling function tags), and it would prove useful to map makers and modders on Java Edition.