SystemTime should not implement ItemArgument

When I wrote !984 (merged) I foolishly assumed that the netdoc format would always encode a timestamp the same way.

However, https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/342-decouple-hs-interval.md introduces a parameter in proper ISO8601 (without timezone) format (with T instead of a space). (It calls it IsoTimeNospace which isn't defined anywhere.) There maybe other places. See torspec#182

I think we probably ought to add .arg_time_old and .arg_time_new or some such.