add dummy routes and handlers and update makefile

service_a/internal/router/router.go

@@ -0,0 +1,16 @@
+package router
+
+import (
+	"fmt"
+	"net/http"
+)
+
+func SetupRoutes() *http.ServeMux {
+	mux := http.NewServeMux()
+
+	mux.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
+		fmt.Fprintf(w, "hello world")
+	})
+
+	return mux
+}