[WIP] Settings model with both pydantic 1 and 2 support

[ijpulidos]

Pydantic 2 changed a significant amount of things in the validators. A quick (and dirty?) way of supporting both is using pydantic.v1 import and catching an ImportError exception.

This is still a work in progress and maybe we might consider just sticking with pydantic 1 for now.

[codecov-commenter]

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[richardjgowers]

I think this is what we're doing too everywhere. If we wanted to migrate to v2 only, we'd have to chase upstream to check if anything is v1 only. I haven't had time to check, but I'd guess that OpenFF + QCStack might be a place where there's significant usage of pydantic (and I have no idea if they've prioritised this).

[ijpulidos]

Closing this. We will be implementing this in a different way now that this is available https://docs.pydantic.dev/latest/migration/#continue-using-pydantic-v1-features