JWKs, ES256K, Hunter, OpenSSL 3.0.0

New Features

• Preliminary JWKs support by @tchinmai7 💟
• Implemented the "alg" ES256K
• Adds support for Hunter Package Manager.

Improvements

• jwt::verifier has been refactored to accept a more generic "verification operation". There are no API breaks.
• Support for OpenSSL 3.0.0 has been confirmed. 🤗 Thanks to @kleinmrk
• Adjustments to the RSA implementation by @akyidrian for wider compatibility

Bug Fixes

• Fixes to_lower that was actually doing a to_upper.

Full Changelog: v0.5.1...v0.6.0-rc.1