flow: implement new methods, add Async ExecutionOption

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

flow/options.go

@@ -124,6 +124,8 @@ type ExecuteOptions struct {
 	Reverse bool
 	// Timeout for execution
 	Timeout time.Duration
+	// Async enables async execution
+	Async bool
 }
 
 type ExecuteOption func(*ExecuteOptions)
@@ -170,6 +172,12 @@ func ExecuteTimeout(td time.Duration) ExecuteOption {
 	}
 }
 
+func ExecuteAsync(b bool) ExecuteOption {
+	return func(o *ExecuteOptions) {
+		o.Async = b
+	}
+}
+
 func NewExecuteOptions(opts ...ExecuteOption) ExecuteOptions {
 	options := ExecuteOptions{
 		Client:  client.DefaultClient,