fix: [att&ck converter] allow multiple external IDs

- There are in some cases external ID references to CAPEC in addition to ATT&CK in techniques
- convert external ID to a list rather than a single string

- as reported by @SYNchroACK
- as hurried along by a disappointed @deresz

tools/mitre-cti/v2.0/create_mitre-enterprise-attack-attack-pattern_galaxy.py

@@ -25,11 +25,12 @@ for element in os.listdir('.'):
             value['value'] = temp['name'] + ' - ' + temp['external_references'][0]['external_id']
             value['meta'] = {}
             value['meta']['refs'] = []
+            value['meta']['external_id'] = []
             for reference in temp['external_references']:
                 if 'url' in reference and reference['url'] not in value['meta']['refs']:
                     value['meta']['refs'].append(reference['url'])
-                if 'external_id' in reference:
-                    value['meta']['external_id'] = reference['external_id']
+                if 'external_id' in reference and reference['external_id'] not in value['meta']['external_id]:
+                    value['meta']['external_id'].append(reference['external_id'])
             value['meta']['kill_chain'] = []
             for killchain in temp['kill_chain_phases']:
                 value['meta']['kill_chain'].append(killchain['kill_chain_name'] + ':enterprise-attack:' + killchain['phase_name'])