digest: implemented ZONEMD generation in server

doc/man/knot.conf.5in

doc/reference.rst

src/knot/conf/schema.c

src/knot/conf/schema.h

src/knot/dnssec/zone-events.c

src/knot/dnssec/zone-sign.c

src/knot/dnssec/zone-sign.h

src/knot/updates/zone-update.c

src/knot/zone/digest.c

src/knot/zone/digest.h