get user creation date from keycloak and store it in the database

[Panaetius]

right now we don't store the creation (registration) date of a user in the database, and by extension also not the creation_date of the user's namespace.

We should get that from keycloak in the user_sync and store it.

For the full sync, there's a createdTimestamp, for the partial sync we need to find out what property there is on the REGISTER event from keycloak or use the event timestamp.