Feat: Overhaul config using Pydantic

[mrkaye97]

Copy of #296

Overhauling the client config to use Pydantic to get rid of a lot of convoluted parsing + validation code.

Main things here:

• Got rid of the config.yaml stuff since nobody seems to be using that and it makes the code paths significantly more complicated.
• Strict typing for all of the fields with the same defaults + field validators in the cases where there's parsing logic like extracting something from the JWT
• Got rid of the ConfigLoader, since now we can instantiate the class directly
• Enabled mypy for the new config class now that types check out