Make `LSPS5ServiceEvent::SendWebhookNotification::headers` a Vec

diff --git a/lightning-liquidity/src/lsps5/event.rs b/lightning-liquidity/src/lsps5/event.rs
index a9c1052..c122738 100644
--- a/lightning-liquidity/src/lsps5/event.rs
+++ b/lightning-liquidity/src/lsps5/event.rs
@@ -15,7 +15,6 @@ use alloc::vec::Vec;
 use bitcoin::secp256k1::PublicKey;
 
 use lightning::impl_writeable_tlv_based_enum;
-use lightning::util::hash_tables::HashMap;
 
 use super::msgs::LSPS5AppName;
 use super::msgs::LSPS5Error;
@@ -70,7 +69,7 @@ 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)>,
 	},
 }
 
diff --git a/lightning-liquidity/src/lsps5/service.rs b/lightning-liquidity/src/lsps5/service.rs
index f7f5e06..0b5a901 100644
--- a/lightning-liquidity/src/lsps5/service.rs
+++ b/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,
diff --git a/lightning-liquidity/tests/lsps5_integration_tests.rs b/lightning-liquidity/tests/lsps5_integration_tests.rs
index 41af2e8..80707a6 100644
--- a/lightning-liquidity/tests/lsps5_integration_tests.rs
+++ b/lightning-liquidity/tests/lsps5_integration_tests.rs
@@ -17,7 +17,7 @@ use lightning::ln::functional_test_utils::{
 };
 use lightning::ln::msgs::Init;
 use lightning::ln::peer_handler::CustomMessageHandler;
-use lightning::util::hash_tables::{HashMap, HashSet};
+use lightning::util::hash_tables::HashSet;
 use lightning::util::test_utils::TestStore;
 use lightning_liquidity::events::LiquidityEvent;
 use lightning_liquidity::lsps0::ser::LSPSDateTime;
@@ -288,15 +288,20 @@ impl TimeProvider for MockTimeProvider {
 	}
 }
 
-fn extract_ts_sig(headers: &HashMap<String, String>) -> (LSPSDateTime, String) {
+fn extract_ts_sig(headers: &Vec<(String, String)>) -> (LSPSDateTime, String) {
 	let timestamp = headers
-		.get("x-lsps5-timestamp")
+		.iter()
+		.find_map(|(key, value)| (key == "x-lsps5-timestamp").then(|| value))
 		.expect("missing x-lsps5-timestamp header")
 		.parse::<LSPSDateTime>()
 		.expect("failed to parse x-lsps5-timestamp header");
 
-	let signature =
-		headers.get("x-lsps5-signature").expect("missing x-lsps5-signature header").to_owned();
+	let signature = headers
+		.iter()
+		.find(|(key, _)| key == "x-lsps5-signature")
+		.expect("missing x-lsps5-signature header")
+		.1
+		.clone();
 	(timestamp, signature)
 }