Richer Extensibility for routes and clusters

## Problem Statement

Today if you want to extend the route or clusters, you can only do it through the metadata property on each, which is a Dictionary. If you want to be able to have structured data its not possible without you forcing it into a string and then parsing it when needed. There are scenarios like A/B testing, or authenticating with back end servers (not pass thru) where you want to be able to store a structure of data in config, and have it available at runtime on the route/cluster objects.

Design for this feature has been moved to https://github.com/microsoft/reverse-proxy/pull/1796