Filter on data in DCQL

[danielfett]

@decentralgabe I don't see a way to filter on conditional data, has this been excluded intentionally?

For example, requiring that an age field is over 21 is incredibly useful and prevents unnecessary disclosure.

If I have a credential and I'm 20 then I should not need to send the credential (or a subset of its fields) when I could simply determine that I should send nothing.

Without these types of conditionals the query language promotes over-identification.

This is not part of this PR, but there's a plan how to do it — we can use the "transformed claims" syntax from "Advanced Syntax for Claims" from the OpenID eKYC working group.

Originally posted by @danielfett in #266 (comment)