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.

まず、issue にリンクされている OpenAPI 拡張仕様と、ライブラリに既存の getSerializableData() の動作を読みます。キーワード属性がどのように公開され、どのように isset() がそれらに対して使われているかを追跡します。呼び出し側が、報告された PHP エラーなしにハイフンを含む拡張名へ簡単にアクセスでき、既存のシリアライズの workaround も引き続き理解されていれば完了です。

索引モデルが issue の本文から書いたものです。