Refactor config files

[jemrobinson]

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⤴️ Summary

• Move external.api and external.interface into external (unnecessary nesting)
• Separate PulumiStack into PulumiSHMStack and PulumiSREStack (better than changing behaviour with strings)
• Move Azure authentication from mixin to external
• Replaced DotFileSettings with better named BackendSettings
• Store BackendSettings in appdirs config path instead of local directory
• Simplify use of BackendSettings to only be updated in init and read during Config construction
• Convert Config from a dotmap to a nested set of dataclasses (allows type-hinting to work correctly)

🌂 Related issues

Closes #1509

Check #1506 before merging

🔬 Tests

Tested locally

[JimMadge]

Looks good.

I think this config structure makes more sense. Using dataclasses is sensible and will help with validation. (We could use something like pydantic, but I think that is overkill unless you have many dataclass-type classes).

It looks like this has enabled other places of the code to be tidied too.