Allow to set a custom 'header content type' in the `cty` field

[themasch]

rfc7519 specifies only one valid value for the cty field in the header: "JWT", meaning the payload itself is another complete JWT. This change supports setting cty to something else (any string) by constructing the header with Some(HeaderContentType::Custom("...")).

This should solve #67. The implementation here comes with a few (minor?) problems:

• HeaderContentType is no longer Copy, since it now contains a String. I was not able to prevent this without changing the public interface. As far as I can see this only means we have to do a clone operation whenever a HCT::Custom is copied (i.e. JoseHeader::content_type). Cloning HeaderContentType::JsonWebToken should - in my understanding - still by a copy.
• It needs complete serde::Deserialize and serde::Serialize implementations, since I was not able to find a way to make serde flatten the the Custom enum variant in any other way.

... so one could argue, every change this PR introduces is a problem?

But maybe it solves a real use case for an acceptable cost?