Issue #3352503: Allow HTTP fetcher to make POST calls

Closes #3352503

src/Plugin/migrate_plus/data_fetcher/Http.php

@@ -61,6 +61,9 @@ class Http extends DataFetcherPluginBase implements ContainerFactoryPluginInterf
     // Ensure there is a 'headers' key in the configuration.
     $configuration += ['headers' => []];
     $this->setRequestHeaders($configuration['headers']);
+    // Set GET request-method by default.
+    $configuration += ['method' => 'GET'];
+    $this->configuration['method'] = $configuration['method'];
   }
 
   /**
@@ -101,7 +104,8 @@ class Http extends DataFetcherPluginBase implements ContainerFactoryPluginInterf
       if (!empty($this->configuration['request_options'])) {
         $options = NestedArray::mergeDeep($options, $this->configuration['request_options']);
       }
-      $response = $this->httpClient->get($url, $options);
+      $method = $this->configuration['method'] ?? 'GET';
+      $response = $this->httpClient->request($method, $url, $options);
       if (empty($response)) {
         throw new MigrateException('No response at ' . $url . '.');
       }