Move proxy and plugin status from shell to Go

In the process we stop printing out the `PLUGIN_FULL_NAME`, but that
is assumed to be something the user can figure out from `docker plugin ls`

proxy/status.go

@@ -0,0 +1,15 @@
+package proxy
+
+type Status struct {
+	Addresses []string
+}
+
+func NewStatus(proxy *Proxy) *Status {
+	if proxy == nil {
+		return nil
+	}
+	status := &Status{
+		Addresses: proxy.normalisedAddrs,
+	}
+	return status
+}