user Pin
OID4VCI: OPTIONAL. String value containing a user PIN. This value MUST be present if user_pin_required was set to true in the Credential Offer. The string value MUST consist of maximum 8 numeric characters (the numbers 0 - 9). This parameter MUST only be used, if the grant_type is urn:ietf:params:oauth:grant-type:pre-authorized_code.