Make LSPS5ServiceEvent::SendWebhookNotification::headers a Vec

While HTTP headers should be a unique K->V mapping, returning three
headers to a user in an event via a `HashMap` is substantially
overkill (and also not trivial to do in bindings). Instead, we
expose them as a `Vec`.

Author: TheBlueMatt

lightning-liquidity/src/lsps5/event.rs

@@ -70,7 +70,7 @@ pub enum LSPS5ServiceEvent {
 		/// - `"x-lsps5-timestamp"`: with the timestamp in RFC3339 format (`"YYYY-MM-DDThh:mm:ss.uuuZ"`).
 		/// - `"x-lsps5-signature"`: with the signature of the notification payload, signed using the LSP's node ID.
 		/// Other custom headers may also be included as needed.
-		headers: HashMap<String, String>,
+		headers: Vec<(String, String)>,
 	},
 }
 

lightning-liquidity/src/lsps5/service.rs

@@ -629,12 +629,12 @@ where
 
 		let signature_hex = self.sign_notification(&notification, &timestamp)?;
 
-		let mut headers: HashMap<String, String> = [("Content-Type", "application/json")]
+		let mut headers: Vec<(String, String)> = [("Content-Type", "application/json")]
 			.into_iter()
 			.map(|(k, v)| (k.to_string(), v.to_string()))
 			.collect();
-		headers.insert("x-lsps5-timestamp".into(), timestamp.to_rfc3339());
-		headers.insert("x-lsps5-signature".into(), signature_hex);
+		headers.push(("x-lsps5-timestamp".into(), timestamp.to_rfc3339()));
+		headers.push(("x-lsps5-signature".into(), signature_hex));
 
 		event_queue_notifier.enqueue(LSPS5ServiceEvent::SendWebhookNotification {
 			counterparty_node_id,