feat: allow for http adapter param

- Base client class now accepts Http Adapter object as param.
- Add http adapter interface

lib/recurly/base_client.php

@@ -31,7 +31,7 @@ abstract class BaseClient
      * In addition to the options managed by BaseClient, it accepts the following options:
      *  - "region" to define the Data Center connection - defaults to "us";
      */
-    public function __construct(string $api_key, LoggerInterface $logger = null, array $options = [])
+    public function __construct(string $api_key, LoggerInterface $logger = null, array $options = [], HttpAdapterInterface $http_adapter = null)
     {
         $this->_api_key = $api_key;
         if (isset($options['region'])) {
@@ -41,6 +41,11 @@ public function __construct(string $api_key, LoggerInterface $logger = null, arr
             $this->baseUrl = BaseClient::API_HOSTS[$options['region']];
         }
 
+        if (is_null($http_adapter)) {
+            $http_adapter = new HttpAdapter;
+        }
+        $this->http = $http_adapter;
+
         $this->http = new HttpAdapter;
         if (is_null($logger)) {
             $logger = new \Recurly\Logger('Recurly', LogLevel::WARNING);

lib/recurly/http_adapter_interface.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace Recurly;
+
+/**
+* @codeCoverageIgnore
+*/
+interface HttpAdapterInterface
+{
+    public function execute($method, $url, $body, $headers): array;
+}