v3: Redesign `Data` hierarchy

[edan-bainglass]

Following up on a comment made in #7138, we here consider the following restructure:

1. There is only Data
2. Sub-types no longer extend Data directly
3. Data is still unique by node_type per data type (in DB)
4. Sub-types implement (and register via entry-point system):
   i. Utility interface, accessible via Data.value
   ii. Pydantic schemas
5. If sub-type package is not installed:
   i. Data.value raises dedicated message (with instructions)
   ii. Pydantic schema is not accessible, leading to attributes remaining a simple dict

@giovannipizzi