Add allow_self_issued and fix example Presentation Definition

[brentzundel]

How is this different from subject_is_issuer?

[csuwildcat]

Honestly forgot about it, but I think there's still something I'd add to subject_is_issuer: a value of "allowed" or "optional" , because the intent here is to signal that the input being described is not preferred or required to be from them, but merely open to it being so.

[decentralgabe]

Honestly forgot about it, but I think there's still something I'd add a value of "allowed" or "optional" to subject_is_issuer, because the intent here is to signal that the input being described is not preferred or required to be from them, but merely open to it being so.

@csuwildcat this is supported to - can be required or preferred https://identity.foundation/presentation-exchange/#relational-constraint-feature

[csuwildcat]

I'd also update the Note in that section to not be strictly about directing them to self-issue because you say so, but also note that it can be used as a signal that the JSON Schema inputs can be rendered in a form by a wallet for self-input of data. We're planning to do this in our wallet extension.

[csuwildcat]

Closing in favor of new subject_is_issuer value PR