Add extension points on RestRequest to support more REST authorization methods

To use the Skara RestRequest interface to interact with certain REST APIs, we need to extend the ability for how authorization is handled. It's currently possible to supply a lambda that adds an authorization header. I would like this lambda to have access to the Request.Builder so that it's able to read existing headers.

It's also common, at least among cloud providers, to require a SHA-256 hash of any contents (of a POST or PUT) in a header. The name of that header varies among providers however. I'm adding an optional field in the QueryBuilder to declare a header name for such a SHA-256 hash, with the RestRequest will automatically fill in if set.

To support other implementations of pagination, I'm adding another FunctionalInterface for supplying a function for creating a HttpRequest.Builder for any "next" link in a HttpResponse. This has a default implementation based on the current next link logic, which can now be overridden.