Consider making value property optional on the SCO Email Address object

[treyka]

@chrisr3d (from CIRCL) proposed making the value property on the Email Address object (STIX 2.0, Part 4, §2.5) optional to accommodate cases where folks want to share information about the intended target of a spear-phishing campaign specifying the email display name but omitting the actual email address of the intended victim.

The proposal would be to make the value property optional and add normative text that at least one of value or display_name MUST be present.

[gtback]

I don't know how I would do this (possibly using an Identity, or possibly using a pattern that just specifies the display_name, or possibly with the UserAccount object... though user_id is required there), but it feels wrong to represent an email address without the address.

[gtback]

Related: #27

[ikiril01]

To @gtback's point, I'm also wondering if this is something we'd ever create Observed Data instances for, or if it's simply a pattern for only a display_name.

[jordan2175]

We talked about this on the working call on 2019-06-11 and are moving to 2.2