Add LSPS5 webhook service implementation

Implements the LSPS5 webhook registration service that
allows LSPs to notify clients of important events via webhooks.
This service handles webhook registration, listing, removal,
and notification delivery according to the LSPS5 specification.

Some details:

- A generic HttpClient trait is defined so users can provide their own HTTP implementation
- A generic TimeProvider trait is defined with a DefaultTimeProvider that uses std functionality
- Uses URL utils to validate webhook URLs according to LSPS5 requirements
- Uses secure message signing logic from the lightning::util::message_signing module
- Works with the events and messages defined in earlier commits
- Tests will be provided in a future commit

Author: martinsaposnic

lightning-liquidity/src/lsps5/event.rs

@@ -9,11 +9,11 @@
 
 //! Contains bLIP-55 / LSPS5 event types
 
-use crate::lsps0::ser::LSPSDateTime;
 use crate::lsps0::ser::LSPSRequestId;
 use alloc::string::String;
 use alloc::vec::Vec;
 use bitcoin::secp256k1::PublicKey;
+use lightning::util::hash_tables::HashMap;
 
 use super::msgs::LSPS5AppName;
 use super::msgs::LSPS5Error;
@@ -31,7 +31,7 @@ pub enum LSPS5ServiceEvent {
 	///
 	/// When this event occurs, the LSP should:
 	/// 1. Send an HTTP POST request to the specified webhook URL
-	/// 2. Include all provided headers, especially the timestamp and signature headers
+	/// 2. Include all provided headers in the request
 	/// 3. Send the JSON-serialized notification as the request body
 	/// 4. Handle any HTTP errors according to the LSP's retry policy
 	///
@@ -69,22 +69,14 @@ pub enum LSPS5ServiceEvent {
 		///
 		/// This contains the type of notification and any associated data to be sent to the client.
 		notification: WebhookNotification,
-		///  Timestamp of the notification.
-		///
-		/// This timestamp is used for signing the notification and must be within 10 minutes
-		/// of the client's local time for the signature to be accepted.
-		timestamp: LSPSDateTime,
-		/// Signature of the notification using the LSP's node ID.
-		///
-		/// This signature helps the client verify that the notification was sent by the LSP.
-		signature: String,
 		/// Headers to be included in the HTTP POST request.
 		///
-		/// Headers should include:
-		/// - Content-Type (application/json)
-		/// - x-lsps5-timestamp (timestamp in RFC3339 format, e.g., "YYYY-MM-DDThh:mm:ss.uuuZ")
-		/// - x-lsps5-signature (signature of the notification using the LSP's node ID)
-		headers: Vec<(String, String)>,
+		/// This is a map of HTTP header key-value pairs. It will include:
+		/// - `"Content-Type"`: with a value like `"application/json"`.
+		/// - `"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>,
 	},
 }