Add support for JWS Detached

[lrosenthol]

I have a use case where it I need to produce a JWT where the payload data shouldn't be included in the token itself, as described here in the spec.

[Thalhammer]

Note that this method needs no support from JWS libraries, as applications can use this method by modifying the inputs and outputs of standard JWS libraries.

If you want to do this you simply need to split the resulting token on the dot and remove the middle part. Before verification you insert it back.

[lrosenthol]

True, but I ended up doing it in a more efficient (at least to me) way. Changes are in my fork at https://github.com/lrosenthol/jws-jwt-cpp