RBAC: Add a function to delete external service roles (#68317)

* RBAC: Add function to delete external service roles

* Adding a test to the service

* Update pkg/services/accesscontrol/acimpl/service_test.go

Co-authored-by: Ieva <ieva.vasiljeva@grafana.com>

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Co-authored-by: Ieva <ieva.vasiljeva@grafana.com>

pkg/services/accesscontrol/accesscontrol.go

@@ -40,6 +40,8 @@ type Service interface {
 	DeclareFixedRoles(registrations ...RoleRegistration) error
 	// SaveExternalServiceRole creates or updates an external service's role and assigns it to a given service account id.
 	SaveExternalServiceRole(ctx context.Context, cmd SaveExternalServiceRoleCommand) error
+	// DeleteExternalServiceRole removes an external service's role and its assignment.
+	DeleteExternalServiceRole(ctx context.Context, externalServiceID string) error
 	//IsDisabled returns if access control is enabled or not
 	IsDisabled() bool
 }