Confirm how to add `spec` to AssetTree

[sync-by-unito]

┆Issue is synchronized with this Asana task by Unito
┆Created By: Tammy Yang

[sync-by-unito]

➤ Bofu Chen commented:

Note to myself: it's for recording spec version

[sync-by-unito]

➤ Bofu Chen commented:

The comment in the parent task:

I need more time to think about how to adopt Schema Evolution in Asset Tree. The benefit will be that Nit / Commit API will handle schema compatibility in the background without maintaining the "specVersion" field. User and application (e.g., storage backend) do not need to touch the schema version either.