Support other file formats (other than XML VOEvent) to ingest external events from

Currently we can only ingest VOEvent XMl files to create external events via gracedb.create_event. There is already a need ingest events delivered via Kafka that have a .json format, which we are currently creating a workaround by converting to a VOEvent packet here (note this code has not been merged yet and subject to change):

https://git.ligo.org/emfollow/gwcelery/-/blob/dfdd84a97dec60257c4d7bd91d6c0c9442ec3de6/gwcelery/tasks/external_triggers.py#L561-626

Example alert: https://git.ligo.org/emfollow/gwcelery/-/blob/a35b3ba998ab4726f90d5fb3cdf87d365cccbc65/gwcelery/tests/data/kafka_alert_fermi.json

In general we should make a more flexible system to ingest external events as GCN moves towards Kafka, potentially able to add new notice types/formats as needed (e.g. Kamland notices also have a different format, etc.)

I assume we need to make additional parser functions such as populateGrbEventFromVOEventFile and add options to use the and determine which schema is being used here.

I can personally help with this development if needed, especially after the start of O4.