SilcAttributePayload
    silc_client_attribute_add(SilcClient client,
                              SilcClientConnection conn,
                              SilcAttribute attribute,
                              void *object,
                              SilcUInt32 object_size);

    Add new Requsted Attribute for WHOIS command to the client library.
    The `attribute' object indicated by `object' is added and allocated
    SilcAttributePayload is returned.  The `object' must be of correct
    type and of correct size.  See the SilcAttribute for object types
    for different attributes.  You may also get all added attributes
    from the client with silc_client_attributes_get function.

    Requested Attributes are different personal information about the
    user, status information and other information which other users
    may query with WHOIS command.  Application may set these so that
    if someone sends WHOIS query these attributes will be replied back
    to the sender.  The library always puts the public key to the
    Requested Attributes, but if application wishes to add additional
    public keys (or certificates) it can be done with this interface.
    Library also always computes digital signature of the attributes
    automatically, so application does not need to do that.