signout function
Signs the user out and invalidates the accessToken.
Implementation
Future<bool> signout(String username, String password) async {
final payload = {
'username': username,
'password': password,
};
final response = await requestBody(
http.post, _authServerApi, 'signout', payload,
headers: {});
final data = parseResponseMap(response);
return data.isEmpty;
}