Controller support for networked scanners

Claircore's scan controller should have support for opting in to making RPC calls, with the extreme caveats that it entails.

The Scanners should opt in by implementing an additional interface, like:

type RPCScanner interface {
    RPCScan(...) (T, error)
}

This new method will get some shared resources passed in, like an *http.Client that's been configured.