json: add json_calc_encoded_arr_len

Analog to json_obj_encode vs. json_calc_encoded_len which
calculates the object len using json_obj_encode, introduce
json_calc_encoded_arr_len which calculates the length using
json_arr_encode. That is needed when the object to be encoded
is array on the root level.

Signed-off-by: Miika Karanki <miika.karanki@vaisala.com>