Annotate sealed traits to get deterministic ordering in schema

[bobbyrauchenberg]

Because of how Mangnolia works, Avro4s can't guarantee it will process sealed traits in a deterministic order

This means we can end up with a situation where the schema registry contains multiple versions of the same schema because for 2 versions the fields are ordered differently

We can use the Avro4s @AvroSortPriority to fix this (we need to confirm this doesn't mess with the ordering of union fields and break defaults)