Use a URN for protocol identifier instead of an unstructured string

[timcappalli]

As we've learned with OID4VP, there are variations of request types (signed vs unsigned) [#185, openid/OpenID4VP/issues/326], and there will be future versions of the protocol.

As the protocol identifier is essentially "typing" the data payload for downstream parties, we need to support a richer protocol identifier beyond a string.

This proposal is to make protocol identifier a URN. For example:

• Signed Requests in OpenID for Verifiable Presentations v1.0: urn:openid:spec:openid4vp:v1.0:signed
• Unsigned Requests in OpenID for Verifiable Presentations v1.0: urn:openid:spec:openid4vp:v1.0:unsigned
• OpenID for Verifiable Credential Issuance v1.0: urn:openid:spec:openid4vci:v1.0:<blah>

(for VCI, should the type be the two different flows (authz code and preauthz code)?)

These identifiers would still be defined in their respective specifications, but high level requirements would be defined here, including requiring this type of identifier for registry inclusion.

[timcappalli]

Important "min bar" details seem to be:

• protocol name
• protocol version
• payload variants