Add Config File and global Settings

[wk9874]

We are starting to get a large number of environment variables which can be set, and will have lots more in the near future as different model options are added. So have added a option to define these settings in either a config toml file, or still via env vars (with slightly different names)

• Adds pydantic-settings as a dependency
• Defines new Settings object, which has a global instance accessed by all parts of code
• Updates env vars everywhere else in code to use new settings object
• Env vars inside worker left unchanged as these will not have access to settings object on worker nodea
• Adds tests and docs