Add onAuthenticated/onSignIn/`onCreate'

[ScottAgirs]

Thinking about enabling multiple sessions for a user, one idea I had was to as an interim enabler of this functionality we could provide hooks that would allow adding logic at the time of creation NextAuth session by introducing onAuthenticated, onSignIn, and `onCreate' hooks.

This would then allow users to create their own local list type, for example, UserSession that would allow a super flexible and open-minded approach to managing sessions depending on their business logic and needs.

These hooks would also allow for any other arbitrary logic they may want to incorporate.

Alternative - perhaps a better way?

Another option that possibly is simpler and would leverage the existing resolver is to simply pass KeystoneAPI query and db objects to resolver so users could make changes to DB as needed.

The downside of this approach would be that it would be a less granular control over the flow and would happen regardless of whether it is the creation of a User (authenticatedItem) or a login of it.