feat: add associated methods to structs for static values

[hashemix]

📌 Summary

Enhanced the schema by incorporating associated methods into the structs for static values.

For instance, we can now retrieve the method name of messages using an associated method, such as:

let method_name = CancelledNotification::method_name(); // Returns "notifications/cancelled"

Previously, this information was only accessible through instance methods.
This improvement should greatly simplify development across various use cases.