This patch was authored by @WFT and released by @gwynne.
Verify JWS tokens using x5c header parameters (ES256 keys only right now).
This PR is based on @0xTim’s work in #75, but uses the X509_STORE/X509_verify_cert API rather than re-implementing that logic in Swift.
Details on x5c: https://www.rfc-editor.org/rfc/rfc7515#section-4.1.6