Support dynamic redirection policies

This issue is being filed to track one of the four suggestions made in JDK-8184279.

- Redirect: Why is this an enum? This seems to needlessly restrict extensibility of
  the API: For example, (as opposed to Effective Java 1st Edition’s type safe enum
  pattern), it makes it impossible to ever define redirect policies at runtime (e.g.
  based on a list of trusted hosts) because the set of instances is fixed at compile time,