flow: add initial flow dag

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

flow/flow.go

@@ -0,0 +1,7 @@
+// Package flow is an interface used for saga pattern messaging
+package flow
+
+type Step interface {
+	// Endpoint returns service_name.service_method
+	Endpoint() string
+}