AuthenticationArtifacts should contain both private key and certificate in single message

[nkrishk]

As per proto, AuthenticationArtifacts should contain both private key and certificate in single message (it is not oneof message)

https://github.com/openconfig/gnsi/blob/main/credentialz/credentialz.proto#L761-L770

message ServerKeysRequest {
  message AuthenticationArtifacts {
    // Private key to be used by the SSH server.
    // OpenSSH-formatted.
    bytes private_key = 1;
    // SSH Certificate (server's public key signed by the Certificate Authority)
    // to be used by the SSH server.
    // OpenSSH-formatted.
    bytes certificate = 2;
  }

But as per credz test, private key and certificate are added as separate list

https://github.com/openconfig/featureprofiles/blob/main/internal/security/credz/credz.go#L280-L298

need clarity on this, based on this i will modify the test scripts