Add support to suppress @xsi:type from major components

[johnwunder]

@xsi:type is automatically added to major components:

• CourseOfAction
• Indicator
• Incident
• ExploitTarget
• TTP
• ThreatActor
• Campaign

It would be nice to be able to manually suppress this to indicate that the producer is creating a reference rather than a definition. This allows profiles to correctly validate definitions while ignoring references (it checks for xsi:type).

[johnwunder]

Quick note: this feature is no longer necessary for the use case I was thinking of (profiles). Not to say we shouldn't do it, but at least you can take my name off the list of people who are asking for it.