Add configuration variables to set GRB rate in gwcelery

Recently AGILE was decommissioned (see https://gcn.nasa.gov/circulars/35727?query=AGILE&startDate=&endDate=) which contributed to a hard-coded rate in ligo-raven : https://git.ligo.org/lscsoft/raven/-/blob/a1a22ec8ed3cf8c71305cccf6255d95917a5e93d/ligo/raven/search.py#L499-505

To mitigate the need for emergency ligo-raven releases in case new GRB satellites are added (e.g. SVOM in #539 (closed)) or removed (non-zero change for Fermi or Swift in O4) we should move this rate(s) to the configuration file in gwcelery and pass it to the em_rate keyword when calculating the joint FAR.