fixup: Improve LSPS5 message types, validation, and testing

- Replace generic String types with strongly-typed Lsps5AppName and Lsps5WebhookUrl with built-in length and format validation
- Restructure imports to follow one-per-line convention
- Add constants for notification method strings
- Make WebhookNotificationMethod enum more consistent with LSPS5 prefix
- Use explicit serde_json::json and serde_json::Value instead of imports
- Improve code documentation with proper ticks and references
- Add comprehensive test vectors from the BLIP-0055 specification

Author: martinsaposnic