Added backwards compatibility layer

Author: Nyholm

src/Plugin/VersionBridgePlugin.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Http\Client\Common\Plugin;
+
+use Http\Client\Common\Plugin;
+use Http\Promise\Promise;
+use Psr\Http\Message\RequestInterface;
+
+/**
+ * A plugin that helps you migrate from php-http/client-common 1.x to 2.x. This
+ * will also help you to support PHP5 at the same time you support 2.x.
+ *
+ * @author Tobias Nyholm <[email protected]>
+ */
+abstract class VersionBridgePlugin implements Plugin
+{
+    abstract protected function doHandleRequest(RequestInterface $request, callable $next, callable $first);
+
+    public function handleRequest(RequestInterface $request, callable $next, callable $first): Promise
+    {
+        return $this->doHandleRequest($request, $next, $first);
+    }
+}

src/VersionBridgeClient.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Http\Client\Common;
+
+use Http\Client\HttpClient;
+use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
+
+/**
+ * A client that helps you migrate from php-http/httplug 1.x to 2.x. This
+ * will also help you to support PHP5 at the same time you support 2.x.
+ *
+ * @author Tobias Nyholm <[email protected]>
+ */
+abstract class VersionBridgeClient implements HttpClient
+{
+    abstract protected function doSendRequest(RequestInterface $request);
+
+    public function sendRequest(RequestInterface $request): ResponseInterface
+    {
+        return $this->doSendRequest($request);
+    }
+}