Merchant API code sample to add supplemental data source to primary data source.

### Java

    // Copyright 2024 Google LLC
    //
    // Licensed under the Apache License, Version 2.0 (the "License");
    // you may not use this file except in compliance with the License.
    // You may obtain a copy of the License at
    //
    //     https://www.apache.org/licenses/LICENSE-2.0
    //
    // Unless required by applicable law or agreed to in writing, software
    // distributed under the License is distributed on an "AS IS" BASIS,
    // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    // See the License for the specific language governing permissions and
    // limitations under the License.

    package shopping.merchant.samples.productsdatasourcesworkflow.v1;
    import shopping.merchant.samples.datasources.v1.UpdateDataSourceSample;
    import shopping.merchant.samples.utils.Config;

    /**
     * This class demonstrates how to add supplemental DataSources to the primary DataSource's default
     * rule.
     */
    public class AddSupplementalDataSourceToPrimaryDataSourceSample {

      public static void addSupplementalDataSourceToPrimaryDataSource(
          Config config,
          String primaryDataSourceName,
          String firstSupplementalDataSourceName,
          String secondSupplementalDataSourceName)
          throws Exception {

        // Update the primary DataSource's default rule to include both supplemental feeds.
        UpdateDataSourceSample updateDatasource = new UpdateDataSourceSample();
        updateDatasource.updateDataSource(
            config,
            primaryDataSourceName,
            firstSupplementalDataSourceName,
            secondSupplementalDataSourceName);
      }

      public static void main(String[] args) throws Exception {
        Config config = Config.load();
        // The names of the primary and supplemental datasources.
        String primaryDataSourceName = "accounts/{account_id}/dataSources/{datasource_id}";
        String firstSupplementalDataSourceName = "accounts/{account_id}/dataSources/{datasource_id}";
        String secondSupplementalDataSourceName = "accounts/{account_id}/dataSources/{datasource_id}";

        addSupplementalDataSourceToPrimaryDataSource(
            config,
            primaryDataSourceName,
            firstSupplementalDataSourceName,
            secondSupplementalDataSourceName);
      }
    }