chg: [stix2 import] Generic way of dealing with payloads in external file & artifact patterns
chg: [stix2 import] Generic way of dealing with payloads in external file & artifact patterns

- After struggling a lot with the different use
  cases, we ended up with the following process:
    - checking if any file:content_ref is there
      and grouping the content refs features
      together if possible
    - After all the content refs have been parsed,
      we check if there still is some payloads

SteveClement/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,MISP/MISP