feat(core-flows,types,medusa): API to add promotions to campaign (#7277)

what:

- adds an API to add promotions to campaign
- reworks module to perform atomic actions

.changeset/silver-lemons-fly.md

@@ -0,0 +1,7 @@
+---
+"@medusajs/core-flows": patch
+"@medusajs/types": patch
+"@medusajs/medusa": patch
+---
+
+feat(core-flows,types,medusa): API to add promotions to campaign