Add interface for http client adapters.

src/Http/Client/Adapter/Stream.php

@@ -14,6 +14,7 @@
 namespace Cake\Http\Client\Adapter;
 
 use Cake\Core\Exception\Exception;
+use Cake\Http\Client\AdapterInterface;
 use Cake\Http\Client\Request;
 use Cake\Http\Client\Response;
 use Cake\Http\Exception\HttpException;
@@ -24,7 +25,7 @@ use Cake\Http\Exception\HttpException;
  *
  * This approach and implementation is partly inspired by Aura.Http
  */
-class Stream
+class Stream implements AdapterInterface
 {
 
     /**
@@ -63,11 +64,7 @@ class Stream
     protected $_connectionErrors = [];
 
     /**
-     * Send a request and get a response back.
-     *
-     * @param \Cake\Http\Client\Request $request The request object to send.
-     * @param array $options Array of options for the stream.
-     * @return array Array of populated Response objects
+     * {@inheritDoc}
      */
     public function send(Request $request, array $options)
     {

src/Http/Client/AdapterInterface.php

@@ -0,0 +1,28 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (https://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (https://cakefoundation.org)
+ * @link          https://cakephp.org CakePHP(tm) Project
+ * @since         3.7.0
+ * @license       https://opensource.org/licenses/mit-license.php MIT License
+ */
+namespace Cake\Http\Client;
+
+use Cake\Http\Client\Request;
+
+interface AdapterInterface
+{
+    /**
+     * Send a request and get a response back.
+     *
+     * @param \Cake\Http\Client\Request $request The request object to send.
+     * @param array $options Array of options for the stream.
+     * @return \Cake\Http\Client\Response[] Array of populated Response objects
+     */
+    public function send(Request $request, array $options);
+}