Allow a more convenient method of working with "x-" specifications

First Point: Excellent project. I've written an OpenApi loader myself. Yours is much better.

Second Point: The openapi specification of keywords allows for the possibility of extending them with the "x-" syntax: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#specification-extensions

In a project I am working on we have made use of this feature. When using cebe/php-openapi I've run into a problem.

• openapi requires the use of the "x-" syntax
• php interprets "x-" not as a variable (or something similar) but sees the "-" as an operation. Particularly with the function isset()
• cebe/php-openapi relies on the native php convention variable/(class attribute names)
• When using isset() to determine if a keyword has been specified PHP reports an error. I have been able to work around this problem by using getSerializableData(). However it would be nice and more convenient if one could access openapi variables (keywords) which use a "-" as part of their name.

首先阅读 issue 中链接的 OpenAPI 扩展规范，以及库现有的 getSerializableData() 行为。跟踪关键字属性是如何暴露的，以及 isset() 是如何用于这些属性的。当调用方能够方便地访问包含连字符的扩展名称而不会出现所报告的 PHP 错误，并且现有的序列化 workaround 仍然得到理解时，即视为完成。