use x-tension value in annotation mapping

Having an x-tension property with value

    Foo:
      type: object
      properties:
        bar:
          type: integer
          x-foobar: raboof

it would probably be nice if I could pass the value to the annotation.

how wold the mapping look like?

  extensions:
    # the mapping expects the source type to be a `string`. It would need 
    # - an identifier to represent the x-tension value and
    # - an identifier as placeholder for the annotation parameter

    # placeholder could look like this
    x-foobar: ?? @ io.openapiprocessor.annotation.Foobar(value = {value})
 
    # value source type could be this?
    x-foobar: {value} @ io.openapiprocessor.annotation.Foobar(value = {value})

首先，查看 issue 中展示的扩展映射语法，并追踪当前如何生成 x-foobar 注解。确定应如何表示 x-tension 值和注解参数占位符，然后定义哪些映射行为和覆盖范围能够证明该值已传递到生成的注解中。