Support Java 8 java.time API #421

Many modern systems have switched to using the java.time API that was introduced in Java 8. I believe that for maximum compatibility with newly created applications, it would be good to support this API alongside the current java.util.Date usages.

For example, in JWTCreator and JWTCreator.Builder, when dealing with expiration, the logical new API way would be to use a java.time.Instant. That way, you could easily write the following:

JWT.create()
    .withExpiresAt(Instant.now().plusSeconds(60 * 15))

It's not that it's impossible to do that with older Date and Calendar implementations, it's just that those classes are outdated (this article from 2017 talks about that), and it would be nice to use the same time API that other parts of most newly created applications are already using.