Add the ability to return maps and arrays in json format

[fgsch]

This will make possible to retrieve multiple values (e.g. location) in a single lookup and use something like libvmod_jq to extract individual values and/or format it.

[fgsch]

Ground work for this committed in rev 028d0bdd1b73460f17f1a4a75a0d2b50d69ac6f2