public static XECPrivateKey fromBase64EncodedX25519PrivateKey(String base64pk) {
        byte[] rawKeyBytes = Base64.getUrlDecoder().decode(base64pk);
        return fromRawX25519PrivateKey(rawKeyBytes);
    }
Returns the bytes representing the BigInteger of the X25519 public key EC point U coordinate
security-utils/src/main/java/com/yahoo/security/KeyUtils.java