Improvements to subject identifier generation

[esune]

In order to support scenarios where multiple pres_req_conf_id could yield the same value for the subject identifier (i.e.: a field with the same value is chosen as sub, the generated sub based on the credential payload is the same because of the content of the credential).

In order to further "namespace" authentication requests, the subject identifier should be built with @pres_req_conf_id as a suffix, so that each proof-configuration will return a different value for sub regardless of whether the credential contents are the same.