Implement enum json serialization

[dont-know-php]

Hi team, this is a small PR fixing enum serialization.

Prerequisites

zend.exception_ignore_args must be disabled.

Description

PHP doesn't support Pure Enum json serialization: https://www.php.net/manual/en/language.enumerations.serialization.php

If a Pure Enum is serialized to JSON, an error will be thrown.

We will get an serialization error, for example, if we want to write json serialized error trace with arguments into log, because some package classes/methods use enum in arguments.

There are two possible solutions:

1. Using Backed Enum
2. Implement JsonSerializable interface. I chose this option because the package uses Pure Enums.