message: add a function to retrieve the internal PublicKey of the LeaderId
diff: @@ -75,6 +75,12 @@ impl BftLeaderSelection {
  }
  }
  +impl LeaderId {
  + pub fn as_public_key(&self) -> &PublicKey<SIGNING_ALGORITHM> {
  + &self.0
  + }
  +}
  +
  impl property::Serialize for LeaderId {
  type Error = std::io::Error;
  fn serialize<W: std::io::Write>(&self, writer: W) -> Result<(), Self::Error> {
  