Additional fields in SMP to hold public certificate(s) and possibly other information about participant

When two participants exchange encrypted, for example by using an XHE, the SMP could be used to store the public certificate of the receiver for encryption. Currently the Certificate class of the SMP data model is a child of the Endpoint class, however, in this scenario the certificate is not associated with the endpoint but with the participant.

It should be analyzed if additional information items related to the participant could be added as well.