Add LSPS5 webhook client implementation

Implements the client-side functionality for LSPS5 webhook registration,
allowing Lightning clients to register, list, and remove webhooks with LSPs.
This client handler processes responses and verifies webhook notification signatures.

Key features:

- Full client API for webhook registration operations
- Per-peer state tracking for pending requests
- Automatic request timeout and cleanup
- Security validation for webhook URLs
- Notification signature verification
- Some tests are provided but more will come in a future commit

This implementation pairs with the service-side LSPS5 webhook
handler to complete the webhook registration protocol according
to the LSPS5 specification.

Author: martinsaposnic