Rename OpenChannelRequest::is_public to is_announced

Referring to announced/unannounced channels regularly leads to confusion
on what they are and when which should be used. To avoid perpetuating
this confusion, we should avoid referring to announced channels as
'public' in our API.

Here we rename the recently introduced field in `OpenChannelRequest`
(which doesn't break released API), and will align the pre-existing
instances of `is_public` in the following commit (which will break API).

Author: tnull

lightning/src/events/mod.rs

@@ -1316,7 +1316,7 @@ pub enum Event {
 		/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
 		channel_type: ChannelTypeFeatures,
 		/// True if this channel is (or will be) publicly-announced.
-		is_public: bool,
+		is_announced: bool,
 		/// Channel parameters given by the counterparty.
 		params: msgs::ChannelParameters,
 	},

lightning/src/ln/channelmanager.rs

@@ -7423,14 +7423,14 @@ where
 					MsgHandleErrInternal::from_chan_no_close(e, msg.common_fields.temporary_channel_id)
 				)?;
 			let mut pending_events = self.pending_events.lock().unwrap();
-			let is_public = (msg.common_fields.channel_flags & 1) == 1;
+			let is_announced = (msg.common_fields.channel_flags & 1) == 1;
 			pending_events.push_back((events::Event::OpenChannelRequest {
 				temporary_channel_id: msg.common_fields.temporary_channel_id.clone(),
 				counterparty_node_id: counterparty_node_id.clone(),
 				funding_satoshis: msg.common_fields.funding_satoshis,
 				push_msat: msg.push_msat,
 				channel_type,
-				is_public,
+				is_announced,
 				params: msg.common_fields.channel_parameters(),
 			}, None));
 			peer_state.inbound_channel_request_by_id.insert(channel_id, InboundChannelRequest {