lib: json: add helper macros for arrays of objects

Since JSON_OBJ_DESCR_ARRAY is suitable only for arrays of primitives,
add JSON_OBJ_DESCR_OBJ_ARRAY (and a ..._NAMED variant), to allow users
to handle arrays of objects.

Having a macro is important, given the unintuitive space optimization
used for storing the offset to the structure element containing the
number of elements in the array.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>