Make topology implement the API client interface

[guicassolato]

For convenience when navigating a topology to find or list resources, the Topology struct, as well as the collection type, could implement a more widely known Client interface for reading resources otherwise from the API server.

controller-runtime's Reader can be a good candidate for this. client-go's ResourceInterface and NamespaceableResourceInterface can be alternatives although these are less specific.

This capability would be especially useful to adapting existing logic to work with the Policy Machinery.

Suggested by @philbrookes