Add custom feeds through API

[SteveClement]

https://github.com/MISP/MISP/issues/3624

Those few links should help:

Add feeds via API: https://github.com/MISP/PyMISP/blob/master/examples/add_feed.py

Generate feeds: https://github.com/MISP/PyMISP/tree/master/examples/feed-generator https://github.com/MISP/PyMISP/tree/master/examples/feed-generator-from-redis

[chrisinmtown]

This issue is fairly cryptic. For me a "custom" feed is a data source that cannot be parsed by the MISP/Cake implementation, so there's no point in adding a Feeds table entry for it.