Pydantic Schemas PR

[multiplemonomials]

Hi all! Just wanted to post a quick update about my newest PR. This PR refactors the JSON configuration schemas to use a Pydantic schema file to lead each configuration file. It's an incremental change, but it should make it easier to understand what is valid in each config file and take us on a path toward mbed_app.json not being such a wild west of options.

Be sure to check out the schema file which, for the first time ever, contains full, complete documentation on what options are allowed in each config file!