Support sync of a single user-storage mapper

[JeanArhancet]

Hey!

Unless I'm missing something, there's no method to sync an individual user-storage mapper, only the provider-level sync via URL_ADMIN_USER_STORAGE.

The Keycloak endpoint:

POST admin/realms/{realm-name}/user-storage/{id}/mappers/{mapper-id}/sync?direction={fedToKeycloak|keycloakToFed}

Defined here: https://github.com/keycloak/keycloak/blob/main/integration/admin-client/src/main/java/org/keycloak/admin/client/resource/UserStorageProviderResource.java#L84

Happy to send a PR if you want!