fix: scim user query endpoint don't allow SortBy custom field (#9235)

# Which Problems Are Solved
- scim list users endpoint (`GET /scim/v2/{orgId}/Users`): handle
unsupported `SortBy` columns correctly

# How the Problems Are Solved
- throw an error if sorting by an unsupported column is requested

# Additional Context
Part of #8140

internal/api/scim/integration_test/users_list_test.go

@@ -93,6 +93,14 @@ func TestListUser(t *testing.T) {
 			wantErr:   true,
 			errorType: "invalidValue",
 		},
+		{
+			name: "custom sort field",
+			req: &scim.ListRequest{
+				SortBy: gu.Ptr("externalid"),
+			},
+			wantErr:   true,
+			errorType: "invalidValue",
+		},
 		{
 			name: "unknown filter field",
 			req: &scim.ListRequest{