The Proxy API service lacked introspection of its internal state.
Introduce a new gRPC Discovery API, implemented by two servers:
1) Proxy API Server: returns a snapshot of discovery state
2) Public API Server: pass-through to the Proxy API Server
Also wire up a new `linkerd endpoints` command.
Fixes#2165
Signed-off-by: Andrew Seigner <siggy@buoyant.io>
Andrew Seigner