sh/jshn.sh: add json_for_each_item()

Function usefull to iterate through the different elements of an
array or object; the provided callback function is called for each
element which is passed the value, key and user provided arguments.
For field types different from array or object the callback is called
with the retrieved value.

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>