build method
Implementation
Future<AuthGoogle> build() async {
String readJson = await getReadJson();
Map jsonData = json.decode(readJson);
var _credentialsResponse = ServiceAccountCredentials.fromJson(readJson);
var data = await clientViaServiceAccount(_credentialsResponse, scope);
_projectId = jsonData['project_id'];
_credentials = data.credentials;
return this;
}