Remove hard resolver dependency on the gRPC registry client.

The most significant change is the introduction of a new Source
interface, which represents a single source of cache entries. In the
existing implementation, the cache was hardcoded to fetch content from
the operator-registry gRPC API on misses. This was a barrier to
off-cluster tools and other applications that should be able to
perform resolution without running a registry server.

A number of test fixtures were affected (for the better) because it's
now possible to directly configure the real Cache implementation to
retrieve test content without using of test doubles.

Signed-off-by: Ben Luddy <[email protected]>