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，檢視 issue 中顯示的擴充功能映射語法，並追蹤目前如何產生 x-foobar 註解。確定應如何表示 x-tension 值和註解參數預留位置，然後定義哪些映射行為和涵蓋範圍能證明該值已傳遞至產生的註解中。