Added Elasticsearch proxy example (#1398)

Co-authored-by: István Zoltán Szabó <istvan.szabo@elastic.co>

docs/examples/proxy/utils/authorize.js

@@ -0,0 +1,54 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+// IMPORTANT: this is not a production ready code & purely for demonstration purposes,
+//            we make no guarantees on it's security and stability
+
+'use strict'
+
+module.exports = (req) => {
+  const auth = req.headers.authorization
+  if (typeof auth !== 'string') {
+    return [{
+      error: 'Unauthorized',
+      message: 'Missing authorization header',
+      statusCode: 401
+    }, null]
+  }
+
+  const [type, token] = req.headers.authorization.split(' ')
+
+  if (type !== 'Bearer') {
+    return [{
+      error: 'Unauthorized',
+      message: 'Bad authorization type',
+      statusCode: 401
+    }, null]
+  }
+
+  if (token.length === 0) {
+    return [{
+      error: 'Unauthorized',
+      message: 'Bad authorization token',
+      statusCode: 401
+    }, null]
+  }
+
+  return [null, token]
+}