feat(core-flows,link-modules,modules-sdk): add cart <> promotion link as source of truth (#6561)

what:

- adds promotion cart link
- update steps to create and remove links

.changeset/silly-clouds-kneel.md

@@ -0,0 +1,7 @@
+---
+"@medusajs/link-modules": patch
+"@medusajs/modules-sdk": patch
+"@medusajs/core-flows": patch
+---
+
+feat(core-flows,link-modules,modules-sdk): add cart <> promotion link as source of truth