Add upgrade migration routine for settings

[dkotter]

Is your enhancement related to a problem? Please describe.

Coming out of the large refactoring work done in #611, some changes were made to how settings are saved.

I believe all settings are now stored under new meta keys and any Provider specific settings (like API keys) are now stored under a new array key within those settings.

What this means is that any current ClassifAI users, when they upgrade to the v3 release, all of their settings will be lost. Ideally we should add an upgrade routine that migrates all existing settings into the new structure and probably delete the old settings once done.

Designs

No response

Describe alternatives you've considered

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