Add support for PHP 8 (#671)

* Add support for PHP 8

* Fix composer for individual packages

Author: jderusse

composer.json

@@ -12,7 +12,7 @@
     ],
     "license": "MIT",
     "require": {
-        "php": "^7.2.5",
+        "php": "^7.2.5 || ^8.0",
         "async-aws/dynamo-db": "^0.3"
     },
     "extra": {