[Discuss] Automatically populate event and cloud fields

Extracted from: https://github.com/elastic/package-spec/issues/310#issuecomment-1085497360

@ruflin:

> Can we separate the discussion around data_stream.* fields and event.* fields? The first ones are strictly required to make the data stream naming scheme work. The event ones we made required because of the security solutions but these are not strictly required I think.