Use pydantic-settings to read config from different sources

[vladsavelyev]

Description of feature

Part of #1790 effort.

We already use Pydantic to validate plot configs, command-line config, and the interactive function parameters. However, globally config is still a singleton module, and there is a lot of custom logic to update it from different sources (command line, multiple YAML locations, environment variables).

pydantic-settings sounds like a relevant project to replace that logic, and also structure the config better. It comes with features to load settings from various sources.

Not a very high priority as the custom functions work well, and existing type hints help with validation, so moving it to 1.23.