Validate presence of entity medata keys

[rom1504]

missing in https://github.com/PrismarineJS/minecraft-data/blob/master/data/pc/1.21.6/entities.json

[ItsDrike]

ref: PrismarineJS/mineflayer#3713 (comment)

Summary:

We should add an automated test that enforces the presence of metadataKeys key in entities.json for PC versions starting at 1.19.4 (and higher). This check should be additional to the schema, with the schema alone still having this key as optional.