Migrate from jsonwebtoken to jwt-rustcrypto and Miscellaneous Improvements

[robjsliwa]

1. JWT Backend Migration:
   • Replaced the jsonwebtoken crate with jwt-rustcrypto to improve flexibility and support for RustCrypto-based algorithms.
   • Updated key handling structures (KeyForDecoding, KeyForEncoding) and adjusted the internal algorithm validation logic accordingly.
   • Updated features in Cargo.toml to reflect the new optional jwt-rustcrypto dependency and adjusted the noring and default feature sets.
2. Refactoring and Simplifications:
   • Removed the ring-based conditional code to focus on a single JWT library (RustCrypto).
   • Simplified the KeyForDecoding and KeyForEncoding structs to handle RSA components and PEM formats consistently across features.
3. Validation Improvements:
   • Enhanced the Validation struct to include new convenience methods such as without_expiry(), with_audience(), and with_issuer(), providing a more flexible API for users.
   • Replaced direct uses of no_exp with without_expiry() for clarity and consistency in handling token expiration.
4. Miscellaneous Fixes:
   • Updated the README to reflect the new usage of jwt-rustcrypto and supported algorithms.
   • Addressed linter errors and minor code style issues.
   • Fixed inconsistencies in key generation and conversion methods, particularly in test utilities.
   • Renamed some modules (algorithm to algorithms) and performed other minor refactors to improve code clarity and organization.