use x-tension value in annotation mapping

[hauner]

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})