Use Pydantic for validation and serialisation

[JimMadge]

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⤴️ Summary

• Add Pydantic, replace Schema and Chili for both validation and serialisation
• Use Pydantic BaseModel features to improve validation
  • Ensure selected context is defined
• Redefine Config and ConfigSection* classes using Pydantic BaseModel
  • This is neater than before.
    Less reinventing the wheel of validation/serialisation
    I think it can be improved further by adding dsh config ... commands so that fewer (or no) fields need to be optional
• Dynamically construct determininistic config items
• Reduce redundancy and verbosity of config file
• Restructure validation functions
  • Less functions needed as Pydantic has many
  • Functions now operate as Pydantic validators
  • Factory added to build Typer validators from Pydantic validators
• Add tests for Config and ConfigSection* classes

🌂 Related issues

Closes #1658
Contributes to #1640

🔬 Tests

[craddm]

Other than that one comment on the readme, LGTM