Add INTEGRAL SPI-ACS alerts to GCN listener

We've gotten some requests from colleagues at Fermi and Swift that it would be a good idea to add INTEGRAL SPI-ACS alerts to our current system since their rate of detections is fairly large. These don't include localizations, but may be useful to signal a coincidence. Note we may send manually trigger an alert in the case we feel it's warranted.

This could be done by adding the appropriate value here: https://git.ligo.org/emfollow/gwcelery/-/blob/8d2dfe14a3c7b9555a0074112f8729c79bfb51c0/gwcelery/tasks/external_triggers.py#L85