Feat:discount expiration date (#403)

* discount expiration validation and testing

* integration testing

* double quotes

* add iso8601 package

* api testing

* add joi validation of start and end dates as well as valid_duration

* valid_duration column

* service testing

* discount validation in services

* integration test with invalid date interval

* include valid_duration when fetching a discount

* rename variable for clarity

* add test for dynamic discount with expiration date

* remove debug code

* adjust tests to reflect valid_duration being included in default fields

* additional discount update validation

* fixed failing test

* set ends_at on dynamic discount creation

* discount integration tests

* removed unused console.log

* removed validation of dynamic discounts by duration and added ends_at to dynamic discount creation

* integration tests for dynamic discount with and without duration

* optional valid duration for dynamic discounts

* allow nullable dynamic discount durations

* expect assertions

* fix unit test after change to dynamic discounts without duration

* change to date instead of string

* add assertions

* error handling

* addressed feedback

packages/medusa/package.json

@@ -63,6 +63,7 @@
     "glob": "^7.1.6",
     "ioredis": "^4.17.3",
     "ioredis-mock": "^5.6.0",
+    "iso8601-duration": "^1.3.0",
     "joi": "^17.3.0",
     "joi-objectid": "^3.0.1",
     "jsonwebtoken": "^8.5.1",