Add abstract class Item_json_func

Client-server protocol will be extended soon to return extended data type information in result set metadata.
All functions returning JSON will mark themself of having the "json" format.

In order to reuse the code easier, let's introduce a new abstract class Item_json_func and derive all Item_str_func descendants returning JSON (and implemented in item_jsonfunc.h) from this new class.

So the class hierarchy in item_jsonfunc.h will look like this:

Item_str_func

  Item_json_func

    Item_func_json_query

    Item_json_str_multipath

      Item_func_json_extract

      Item_func_json_array_append

        Item_func_json_array_insert

      Item_func_json_insert

      Item_func_json_remove

      Item_func_json_search

    Item_func_json_array

      Item_func_json_object

      Item_func_json_merge

        Item_func_json_merge_patch

    Item_func_json_format

  Item_func_json_value

  Item_func_json_quote

  Item_func_json_unquote

  Item_func_json_type

  Item_func_json_keys

Note, we earlier did the same for other data types, e.g.:

• Item_timefunc
• Item_datefunc
• Item_datetimefunc
• Item_geometry_func
  etc.