AuthN: Introduce DefaultOrgID function for managed service accounts (#93432)

* Managed Service Accounts: Use AutoAssignOrgID

* Fix the IsExternalServiceAccount function

* Reassign service account role

* Account for AutoAssignOrg

* Update pkg/services/serviceaccounts/models.go

* Simplify IsExternalServiceAccount function

* Add tests

* Easier to understand test

* Revert small change

pkg/services/accesscontrol/database/externalservices_test.go

@@ -103,10 +103,10 @@ func TestAccessControlStore_SaveExternalServiceRole(t *testing.T) {
 				{
 					cmd: accesscontrol.SaveExternalServiceRoleCommand{
 						ExternalServiceID: "app1",
-						AssignmentOrgID:   1,
+						AssignmentOrgID:   2,
 						ServiceAccountID:  2,
 					},
-					wantErr: true,
+					wantErr: false,
 				},
 			},
 		},