ZKP Support

[jogu]

As per #1 (comment) ZKP support was removed from the initial proposal and we need to discuss further and create a section for it.

[awoie]

Carrying over from #8, where this came up again. @c2bo noted ZKP likely has implications for the query language, so scoping probably comes before drafting:

• Which ZKP schemes / credential formats are in scope, if any
• Whether ZKP is its own credential format section, or a presentation mode that applies across formats
• Impact on the query language
• Impact on the response structure and transaction transcript binding
• Whether any of this is MVP or deferred

Deliberately not blocking the mdoc and SD-JWT VC sections on this.