lib/plugins/aws/provider/awsProvider.js/AwsProvider/getApiGatewayRestApiRootResourceId
class AwsProvider:  getApiGatewayRestApiRootResourceId() {
    if (
      this.serverless.service.provider.apiGateway &&
      this.serverless.service.provider.apiGateway.restApiRootResourceId
    ) {
      return this.serverless.service.provider.apiGateway.restApiRootResourceId;
    }
    return { 'Fn::GetAtt': [this.naming.getRestApiLogicalId(), 'RootResourceId'] };
  }