To be able to incrementally work on add this new Datatype, we need some minimal Datatype configuration to get started.
This must be behind a disabled-by-default configuration option (T333666: Create feature flag config for new EntitySchema Datatype).
Also, we need to double-check if that can actually exist at the same time as the lagacy Datatype in Wikibase, or if have to do something about that first (T333662: Remove legacy EntitySchema Datatype in WikibaseRepo).
Ideally, this specific minimal config would be as simple as
'PT:entity-schema' => [ 'value-type' => 'string' ],
plus the needed hook-boilerplate.