Support xmap in TableRowType #1153
Description
Scio's BigQueryType supports type overrides via OverrideTypeProvider. This is used heavily by libs like Elitzur to do field-level validation at runtime. However, there is currently not a migration path for case classes with overriden types to migrate to magnolify-bigquery.
We could support this by unsealing the TableRowType trait/adding some kind of xmap method so that users can define their own type overrides in Magnolify.