fixup: Add custom URL parser for LSPS5

Adds a new url_utils.rs module that provides:

- A lightweight URL parser specialized for LSPS5 webhook validation
- A URL trait and Url implementation focusing on scheme and host extraction
- RFC-compliant scheme validation
- Comprehensive test coverage for various URL scenarios
This implementation allows validating webhook URLs without depending on the external url crate

Author: martinsaposnic