Feature Request: Add shortcut variables for simple use cases

[jaredfholgate]

Consider adding shortcut variables for when a principal id and role definition name / id are supplied to support a less verbose option for that scenario.

E.g.

simple_role_assignments = {
  resource_group_contributors = {
     scope = "######"
     role_dfinition_name = "Contributor"
     principal_ids =  [ "####", "####" ]
  }
}