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.