post method
Performs a POST request to the specified url, adding the authorization token.
If no token already exists, or if it is expired, a new one is requested.
Implementation
Future<http.Response> post(String url,
    {Map<String, String>? headers,
    dynamic body,
    http.Client? httpClient}) async {
  return _request('POST', url,
      headers: headers, body: body, httpClient: httpClient);
}