Allow all array elements in JsonPointerBasedFilter

[cowtowncoder]

(note: re-created #1026 against 2.16).

Currently JsonPointerBasedFilter only processes Json Arrays by index. eg /0/id.

With this change it is possible to extract all elements using a pointer in the form of //id:

source:

[
   {
      "id":1,
      "stuff":[
         {
            "name":"first",
            "type":"a"
         },
         {
            "name":"second",
            "type":"b"
         }
      ]
   },
   {
      "id":2,
      "stuff":[
         {
            "name":"third",
            "type":"c"
         },
         {
            "name":"fourth",
            "type":"d"
         }
      ]
   }
]

pointers:

"//id"
"//stuff//name"

result:

[
   {
      "id":1,
      "stuff":[
         {
            "name":"first"
         },
         {
            "name":"second"
         }
      ]
   },
   {
      "id":2,
      "stuff":[
         {
            "name":"third"
         },
         {
            "name":"fourth"
         }
      ]
   }
]