Refactor code to use async/await, modular functions, and improve token handling.

- Switched from callbacks to async/await for clearer, more maintainable code.
- Broke up the monolithic code into smaller and modular functions.
- Refined id token validation logic.
- Changed the internal /_id_token_validation location to /_token_validation.
- Minimum required njs version is 0.7.0 now.

Author: route443