Hook for modifying the fields on AuthUser

[infomiho]

When users get the user in the pages that have authRequired: true or by using the useAuth hook they get the AuthUser object. This user object contains only the fields on the User entity and auth specific identities object.

I think we could provide a hook in Wasp that would allow users to customise the user object: adding new fields (like their role) or removing some sensitive fields (removing hasPaid).

We should be careful not to allow users to break Wasp's auth system while still giving them flexibility. The AuthUser type should reflect the new fields as well.

Related: https://ptb.discord.com/channels/686873244791210014/1340391750665109637/1341865419333570633