Support Shannon Entropy on File / Artifact Objects and not just within Windows™ PE Section Type

[animedbz16]

Looking through Stix 2.1 documentation, it appears that only one section discusses a property of entropy which is located within a Windows™ PE Section Type (See https://docs.oasis-open.org/cti/stix/v2.1/os/stix-v2.1-os.html#_ioapwyd8oimw)

Shannon Entropy is a fundamental metric for measuring randomness in any file, not just Windows PE sections. The restriction of the entropy field to only the Windows PE Section Type in STIX 2.1 (section 6.7.6.3) seems like an arbitrary limitation

It seems more appropriate to move this field to be supported for directly under File and/or Artifact objects.