json_pointer: split json_pointer_set_with_array_cb()

json_pointer: split json_pointer_set_with_array_cb()

JSON patch is a bit more clear on how some array operations should be
handled. Unfortunately, handling them on a case-by-case is a bit tricky
because it's difficult to satisfy properly an 'add' operating with a 'move'
operation and the basic json_pointer_set().

With json_pointer_set{f}() we use json_object_array_put_idx() to insert a
value at a certain index.

With JSON patch:
* for the 'add' operation, we need to insert a value at a given index,
  which means shifting existing values by one to the right
  - also, we cannot allow values to be inserted/added outside the bounds of
    the array
* a 'move' operation, is described as a 'remove' and then an 'add';
  for arrays this complicates things, because when we want to a move a
  value within the array, we have to remove it first (during which the size
  of the array is reduced by one); when the size of the array is reduced by
  one, we can't add it to the last position in the array (before the
  remove)

The only sane method to handle this (after a few considerations) is to
provide a callback to the function that does the final put/insert into
the array. That way, we can do some final checks where these are needed to
handle each corner-case.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>