Ability to add individual role assignments

[CmdrSharp]

Currently, the Grafana Providers grafana.RoleAssignment controls all role assignments for a given role. The terraform provider nowadays also has a RoleAssignmentItem which can add the assignment to a single actor. This functionality should be lifted in, as exhaustively providing all teams/users for which a role is to be added is not always the simple approach.

This seems to have been introduced in 2.18.0 of the Terraform provider.