Refactor Config to use Pydantic `BaseSettings`

[stan-dot]

based on the discussion in #495 this refactor would make the config more standard
#649 (comment)

[stan-dot]

field priorities suggested by Pydantic are - https://docs.pydantic.dev/latest/concepts/pydantic_settings/#field-value-priority

we can customize this if we'd like to a different setup

#495 (comment)

[callumforrester]

I made the title slightly clearer, but yes please