Knot resolver in CU 203: enable HTTP API

[Robin Roevens <robin.roevens@disroot.org>]

Hi all

For Zabbix to be able to get knot-resolver metrics, I would like to have
the HTTP API enabled
(https://www.knot-resolver.cz/documentation/latest/manager-api.html).
Receving the same metrics is currently already possible using kresctl
however this is very slow and seems to use quite some resources
(especially on my mini appliance v1: one cpu goes to 100% for 2s) while 
when using curl to get the metrics from the HTTP API, they are returned
instantly without noticable cpu usage:
Timings for kresctl:
real	0m1.979s
user	0m1.825s
sys	0m0.126s
Timings for curl using the http api:
real	0m0.051s
user	0m0.015s
sys	0m0.018s

And secondly, if I would use kresctl, I would need an additional config to
zabbix_agent and sudoers to support calling that binary, while
using the HTTP API is natively built in into the zabbix_agent and much
more performant.

I have added a patch that will enable the HTTP API in the knot resolver
config. If possible, I would like this to be applied already to CU 203
so that I can release a Zabbix template to monitor knot without the need
for additional changes to the zabbix_agent pak that would then probably
have to wait for at least CU 204.

Regards
Robin

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