Add hex key support

[virusvfv]

In some cases HMAC key for signing JWT is non-ascii hex string. So we have to support it with JWT4B.
For example U can add in KeyHelper class ( in cleankey or some else method) condition: if key begins from 0x... then key string is been interpreted as hex string.

[virusvfv]

I created pull request about it ...

[ozzi-]

😎👍
3d30b57