[Federation] Implement the kubefed command.

Also:
1. Add it to the client build targets list.
2. Register `kubefed join` and `kubefed unjoin` commands.

hack/lib/golang.sh

@@ -95,6 +95,7 @@ fi
 # The set of client targets that we are building for all platforms
 readonly KUBE_CLIENT_TARGETS=(
   cmd/kubectl
+  federation/cmd/kubefed
 )
 readonly KUBE_CLIENT_BINARIES=("${KUBE_CLIENT_TARGETS[@]##*/}")
 readonly KUBE_CLIENT_BINARIES_WIN=("${KUBE_CLIENT_BINARIES[@]/%/.exe}")