Use pydantic models for field/table definitions

[tombola]

In order to reuse the table definitions later as pydantic validated records, first step is to extend pydantic model for the existing classes, whilst ensuring existing table and field name lookups continue to work.

For lowest cost implementation, this is done by using the default value of the pydantic field, though would hope to make use of field aliases at a later point.

[tombola]

I want to get this merged soon as possible to avoid other branches diverging 🤞