Add support for --minsecs htgettoken option

Quoting from https://git.ligo.org/computing/sccb/-/issues/1758#note_1388235:

It was not clear to me, though I may have just missed it, whether this utility does anything to make sure that the bearer token should remain alive until the next time the robot auth runs, by for instance setting minsecs. I know that PyCBC got bitten by this with their home-brewed version of this functionality.

Users of igwn-robot-get should be able to specify --minsecs as passed on to htgettoken to ensure that each time the tool runs under cron (or similar scheduler) it will get a new token that should be valid at least until the next time the thing runs.