[WIP] Replace Spec classes with Pydantic models

[rly]

Replaces #1273.

Start work on replacing docval with Pydantic (#1129) with the Spec classes, where are at the bottom of the stack and have relatively few interdependencies.

Working through this exercise highlights what we need to start deprecating/changing in the current API to migrate toward using Pydantic internally.

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?