Adding support for the protected property

[lukehedger]

The JWS protected property is defined here as:

"protected", with the value BASE64URL(UTF8(JWS Protected Header))

Compared to the header property:

"header", with the value JWS Unprotected Header

Also see an example JWS JSON Serialization.

This library currently encodes the header and does not support protected.

[kantai]

Interesting @lukehedger -- this would require a little bit of effort to include support for, but it should be doable. I'd happily review a PR on develop to add support here, but otherwise, it would take us sometime to implement.