Support `x-mitre-asset`

[mehaase]

Per Jared:

Also, v14 introduced a new STIX type - x-mitre-asset that is currently only found in the ICS domain. Not sure how you want to handle that, but it might go here: https://github.com/center-for-threat-informed-defense/attack-sync/blob/attack14/src/attack_sync/site_builder.py#L62-L70

You can see the Asset files here: https://github.com/mitre/cti/tree/master/ics-attack/x-mitre-asset