Feat(medusa, user, core-flows): User creation with invites (#6413)

**What**
- add accept invite endpoint 

**Invite accept flow**
- authenticate using the auth endpoint to create and auth-user
- invoke accept endpoint with token and info to create user

integration-tests/plugins/medusa-config.js

@@ -48,6 +48,9 @@ module.exports = {
       scope: "internal",
       resources: "shared",
       resolve: "@medusajs/user",
+      options: {
+        jwt_secret: "test",
+      },
     },
     [Modules.STOCK_LOCATION]: {
       scope: "internal",