Add properties to authentication token

[pulphix]

We are migrating from a different authentication system, and it would be helpful to have a simple way to map the previous user ID to a property within the JWT, such as external_user_id. This approach would allow us to avoid creating an additional table in our application database and eliminate the need for frequent database calls.

[isaachinman]

Similarly to this, I need to be able to set aud within the AuthKit JWTs, which does not seem possible.

[aygl0619]

We are migrating from a different authentication system, and it would be helpful to have a simple way to map the previous user ID to a property within the JWT, such as external_user_id. This approach would allow us to avoid creating an additional table in our application database and eliminate the need for frequent database calls.