Cleanup introspection interface

Split service proxy from service plugin.
Make introspection service easier for clients to use.
Update service proxy to support grpc and ttrpc.

Signed-off-by: Derek McGowan <derek@mcg.dev>

cmd/ctr/commands/plugins/plugins.go

@@ -71,7 +71,7 @@ var listCommand = &cli.Command{
 		}
 		defer cancel()
 		ps := client.IntrospectionService()
-		response, err := ps.Plugins(ctx, context.Args().Slice())
+		response, err := ps.Plugins(ctx, context.Args().Slice()...)
 		if err != nil {
 			return err
 		}