homectl: split out parser parse_ssh_authorized_keys

homectl: split out parser parse_ssh_authorized_keys

The idea is to split out helpers to make parse_argv() itself more
manageable. If possible, the helpers will be reused in many places. But
even if not, as in this case, it seems good to split out the code for a
specific option. Always, the sd_json_variant** pointer or pointers that
are operated on are selected in the caller. This way it is easier to see
which of the identity variables is used and if two different ones are
used.