add an API endpoint for user creation

in the context of migrating nextcloud user data, we need to

• create users in keycloak with a unique username.
• create users in laboite with the same username, firstname, lastname and email as in keycloak through an API call
• send user nclocator and nctoken to migration script through an API call (based on user email).

To make these accounts compatible with keycloak authentication, laboite will need to match users on their username (preferred_username in keycloak vector) if no keycloak id is matching.