Add sunssActor to control exposures based on running Gen2 status

Add a new actor which pays attention to the gen2Actor traffic and decides when to slew SuNSS and start and stop SPS visits, and when to keep or discard the exposures.

The strawman is simply to run SuNSS whenever the telescope starts guiding, and to finish and keep SPS visits if the telescope stays on that field for "long enough". If not long enough, simply abort and discard the visit.