Allow php-http/discovery as a composer plugin (laravel#6106)

Author: nicolas-grekas

composer.json

@@ -57,7 +57,8 @@
         "preferred-install": "dist",
         "sort-packages": true,
         "allow-plugins": {
-            "pestphp/pest-plugin": true
+            "pestphp/pest-plugin": true,
+            "php-http/discovery": true
         }
     },
     "minimum-stability": "stable",