json: Add top-level array encoding support

The library supports the declaration of JSON arrays as both nested and
top-level elements. However, as the provided encoding functions
json_obj_encode() and json_obj_encode_buf() interpret all input
structures as objects, top-level arrays are encoded as

{"<field_name>":[{...},...,{...}]}

instead of

[{...},...,{...}].

Add new functions json_arr_encode() and json_arr_encode_buf() that
enable top-level JSON array encoding.

Signed-off-by: Markus Fuchs <markus.fuchs@de.sauter-bc.com>