message: Add respond on asynk::Message
diff: -use std::fmt;
  +use std::{
  + fmt,
  + io,
  +};
  use crate::{asynk::client::Client, Headers};
  @@ -21,6 +24,19 @@ pub struct Message {
  pub(crate) client: Client,
  }
  +impl Message {
  + /// Respond to a request message.
  + pub async fn respond(&self, msg: impl AsRef<[u8]>) -> io::Result<()> {
  + match self.reply.as_ref() {
  + None => Err(io::Error::new(
  + io::ErrorKind::InvalidInput,
  + "no reply subject available",
  + )),
  + Some(reply) => self.client.publish(reply, None, None, msg.as_ref()).await,
  + }
  + }
  +}
  +
  impl fmt::Debug for Message {
  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> Result<(), fmt::Error> {
  f.debug_struct("Message")
  