add: [stix2 import] Importing external User Account objects
add: [stix2 import] Importing external User Account objects

- Reusing parsing functions

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