Make Topology Instance Available in WebContext for Runtime Access

There are times when access to the topology and the configured services within it is required by an application, service or provider at runtime.

For instance, we need to be able to check the whitelist expression that is configured in the KnoxSSO service from within the KnoxAuth application. Rather than redundantly configuring it in both places or making a topology specific config item global within gateway-site.xml we should provide access to the topology object itself for all apps, services, providers processing requests within a topology.