Fix Base64URL JWK conversion for ECDSA keys
This patch was authored by @MFranceschi6 and released by @0xTim.
Fixes an issue where JWTKit does not follow RFC7517
An jwk representing an ecdsa key is represented with the parameters x and y in base64url-encoded form. Previously the parameters were converted as Base64 encoded instead of Base64URL encoded.
Adds new APIs for converting to and from base64 URL encoded data