Better way to resolve `allOf`

[SOHELAHMED7]

Though PR SOHELAHMED7#30 and SOHELAHMED7#14 fixes corresponding issues, we need a more proper way for how allOf is resolved.

This can be part of this lib or cebe/php-openapi.

[SOHELAHMED7]

More specifically if the property have reference.

Example:

invoice:
          allOf:
            - $ref: '#/components/schemas/Invoice'
            - x-faker: false
            # - nullable: true


user_2:
            allOf:
              - $ref: '#/components/schemas/User'
              - x-fk-on-update: CASCADE
              - x-fk-on-delete: SET NULL