[FEATURE] Implement group settings for extensions

[dbwiddis]

Is your feature request related to a problem?

Part of #148

A Group Setting is primarily just a validator function for elements of a Settings object.

Implementation should be trivial once the functional interface capability from #154 is implemented.

What solution would you like?

A new SettingType for Settings will be required to handle these, but otherwise should parallel the other implementations.

What alternatives have you considered?

None, as the approach here seems obvious. Open to ideas!

Do you have any additional context?

https://xkcd.com/2432/