Use pydantic to validate config

[eggmaster]

We can use pydantic to ensure that the expected config values are present. This is an improvement over the existing use of 'assert' statements.

Also adds some unit tests for the new validation.

[jpichon]

Tiny nit question inline, otherwise lgtm! As someone not very familiar with pydantic yet, I really appreciate the tests to illustrate what the options and flags mean. It's neat.

[jguiditta]

Aside from feeling like dict-naming in some of the tests is a bit confusing, overall, I like the changes. Good abstraction to avoid repetition and lock down the validation, along with thorough tests to show how it all works!