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

packages/user/integration-tests/utils/get-init-module-config.ts

@@ -10,6 +10,7 @@ export function getInitModuleConfig() {
         schema: process.env.MEDUSA_USER_DB_SCHEMA,
       },
     },
+    jwt_secret: "test",
   }
 
   const injectedDependencies = {}