Add Alternative_IDs to ObservableType

[johnwunder]

This would serve the same purpose as Alternative_ID on Incident and External_ID on Incident.

(suggested by someone who wanted to remain anonymous)

[johnwunder]

Personally, would suggest normalizing this field and including it across all top-level STIX and CybOX constructs. See STIXProject/schemas#358

[athiasjerome]

I suggest reviewing 8.3 Conceptual Threat Risk Model::Core Concept Library::Identifiers (Page 40) of
http://www.threatrisk.org/spec/RevisedSubmission/Revised%20Operational%20Threat%20Risk%20Submission.pdf