keymgr: support importing public-only key

doc/man/keymgr.8in

doc/man_keymgr.rst

src/dnssec/lib/dnssec/key.h

src/dnssec/lib/key/key.c

src/dnssec/shared/keyid_gnutls.c

src/knot/dnssec/key-events.c

src/knot/dnssec/zone-keys.c

src/utils/keymgr/functions.c

src/utils/keymgr/functions.h

src/utils/keymgr/main.c