module: add DNSSEC signing API + adapt module onlinesign

src/knot/dnssec/zone-sign.c

src/knot/dnssec/zone-sign.h

src/knot/include/module.h

src/knot/modules/onlinesign/onlinesign.c

src/knot/nameserver/query_module.c

src/knot/nameserver/query_module.h