Add verification property to external references

[jordan2175]

Originally we had implemented a limited version of the STIX design for a hash property. However, it became clear that without extra details, there is no good way of implementing that. Meaning do you include the headers, or just the content. Is it the whole resource or just part of it. So we dropped this from the spec for now but need to add it back later when we do digital signatures.

[allant0]

In addition, we may wish to address the verification aspect more holistically (as defined in the requirements specification) where we define that integrity of playbook sharing is important requirement. The topic of integrity of a resource within cacao is not limited to this specific object.