Add ability to proxy to discovery server in another environment

To simplify creating test environments, want to be able to configure a discovery server with a parent environment, a set of discovery servers in that parent environment, and a set of service types.

The discovery server should refuse announcements for any service types in the set.

Any discovery queries for any service types in the set should return the same answers as the discovery servers in the parent environment do.

This allows clients in the test environment to consume a configured list of services from a parent environment instead of requiring an additional deployment of those services and their dependencies in the test environment.