Implement bidirectional backfilling #5269
Description
In #5257 we are implementing backfilling/renaming of deprecated attributes. "Backfilling" in this context means that if "old.attribute" is deprecated, its replacement is "new.attribute", and its status is "backfill", then Relay will copy the value of "old.attribute" and also store it under "new.attribute".
However, it may be useful to also support the reverse. That way, SDKs could already switch to sending the updated attribute and product logic referring to the deprecated attribute would still work. In principle this could be implemented in Relay alone (by looking up aliases), but it might be worth to update the schema in conventions to account for this use case.