Whitelisting of valid users for an app

## Is your feature request related to a problem? Please describe
For now, we have solved the need for control of which persons and/or organisations that can use an app by letting the app owner restrict instantiation so that only the app owner can instantiate. Some cases does however need the end user to be able to instantiate with the same control over which users are valid.

## Describe the solution you'd like
The app owner should be able to define a whitelist of valid org.no-s or SSNs for the app. When instantiating, the app should check if users are in the whitelist. As the whitelist would contain SSNs, we cannot host it in a way that allows access for other than the app and the administrators of the app owner.

## Describe alternatives you've considered
Demanding that app owners who need this host their own whitelist and implement instantiation logic that includes an external check.

## Additional context
This is one of the main use cases of the "abonnement" functionality in Altinn II.

No files, tests, or entry points are named. Start by reviewing app instantiation and the existing "abonnement" functionality, then define how a protected whitelist of org.no-s or SSNs would be configured and checked; done means unauthorized users cannot instantiate while valid whitelisted users can.

Written by the indexing model from the issue text.