NS1: add support for REDIRECT

[costasd]

It seems that NS1's REDIRECT is becoming more mature these days (REDIRECTs are returned through the API for example - see #3167) and there's some support about it in the NS1 go client already.

Given that URLFWD has gone away, and with REDIRECT coming in as the replacement, check if all the missing bits are there now (=docs, API, SDK) and implement it into the NS1 provider.