Expose ChannelMonitor::get_counterparty_node_id

This fixes an oversight in ac842ed
namely that it left users unable to implement their own
`ChainMonitor` from outside of the `rust-lightning` crate.

Author: TheBlueMatt

lightning/src/chain/channelmonitor.rs

@@ -1214,7 +1214,11 @@ impl<Signer: Sign> ChannelMonitor<Signer> {
 		self.inner.lock().unwrap().get_cur_holder_commitment_number()
 	}
 
-	pub(crate) fn get_counterparty_node_id(&self) -> Option<PublicKey> {
+	/// Gets the `node_id` of the counterparty for this channel.
+	///
+	/// Will be `None` for channels constructed on LDK versions prior to 0.0.110 and always `Some`
+	/// otherwise.
+	pub fn get_counterparty_node_id(&self) -> Option<PublicKey> {
 		self.inner.lock().unwrap().counterparty_node_id
 	}