update composer.json to include ext-json

Author: Dominik Liebler

composer.json

@@ -10,7 +10,8 @@
     "minimum-stability": "stable",
     "require": {
         "php": ">=7.2",
-        "psr/http-message": "^1.0"
+        "psr/http-message": "^1.0",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^8",