Management api set up webhook

[lojzatran]

TODO:

• [ ] improve the code - add logs, verify error checkings, clean up of unnecessary lines
• [ ] fix tests
• [ ] add tests for both ensure methods
• [ ] manually test that the webhooks and HMAC work after automatic registration
• [ ] update docs - add new params apiManagementUrl, generateHmacKey, notificationModuleBaseUrl, add a note that HMAC and webhooks could be automatically registered