Future-proof the metadata schema

[mildbyte]

https://github.com/splitgraph/seafowl/blob/f00efc451aaa80a818b42e5d0be72efe39f3f50c/migrations/20220705072248_create_tables.up.sql

Make sure that, when we need to, we can migrate:

• the table storage format (e.g. tag a table with "v1" and let users upgrade the table)
• region indexes
• the type storage format (currently we just dump an Arrow JSON string, we should have scope to add other custom datatypes later)