Group Webhooks for Subgroups - Add, Delete
### Release notes
GitLab has made it easier for you to build subgroup management automation with a subgroup webhook. This webhook is triggered when a subgroup is created, updated, or deleted. Automation can now be built without relying on continuous API calls, which is cumbersome and puts an unnecessary performance load on your GitLab instance.
### Problem to solve
Customers want to know when subgroups are created so they can trigger automated processes.
For example:
* Ensure that the group has settings that adhere to their organization's standards.
* Capture the group creation in their external systems.
* Automatically add users to groups that follow an inner sourcing model.
### Intended users
* [Sidney (Systems Administrator)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sidney-systems-administrator)
<!--### User experience goal
<!-- What is the single user experience workflow this problem addresses?
For example, "The user should be able to use the UI/API/.gitlab-ci.yml with GitLab to <perform a specific task>"
https://about.gitlab.com/handbook/engineering/ux/ux-research-training/user-story-mapping/ -->
### Proposal
* Create a group webhook for Subgroups
* Trigger events when
* Subgroups are created.
* Subgroups are deleted.
### Further details
<!-- Include use cases, benefits, goals, or any other details that will help us understand the problem better. -->
### Permissions and Security
<!-- What permissions are required to perform the described actions? Are they consistent with the existing permissions as documented for users, groups, and projects as appropriate? Is the proposed behavior consistent between the UI, API, and other access methods (e.g. email replies)?
Consider adding checkboxes and expectations of users with certain levels of membership https://docs.gitlab.com/ee/user/permissions.html
* [ ] Add expected impact to members with no access (0)
* [ ] Add expected impact to Guest (10) members
* [ ] Add expected impact to Reporter (20) members
* [ ] Add expected impact to Developer (30) members
* [ ] Add expected impact to Maintainer (40) members
* [ ] Add expected impact to Owner (50) members -->
### Documentation
<!-- See the Feature Change Documentation Workflow https://docs.gitlab.com/ee/development/documentation/workflow.html#for-a-product-change
* Add all known Documentation Requirements in this section. See https://docs.gitlab.com/ee/development/documentation/feature-change-workflow.html#documentation-requirements
* If this feature requires changing permissions, update the permissions document. See https://docs.gitlab.com/ee/user/permissions.html -->
### Availability & Testing
<!-- This section needs to be retained and filled in during the workflow planning breakdown phase of this feature proposal, if not earlier.
What risks does this change pose to our availability? How might it affect the quality of the product? What additional test coverage or changes to tests will be needed? Will it require cross-browser testing?
Please list the test areas (unit, integration and end-to-end) that needs to be added or updated to ensure that this feature will work as intended. Please use the list below as guidance.
* Unit test changes
* Integration test changes
* End-to-end test change
See the test engineering planning process and reach out to your counterpart Software Engineer in Test for assistance: https://about.gitlab.com/handbook/engineering/quality/test-engineering/#test-planning -->
### What does success look like, and how can we measure that?
<!--
Define both the success metrics and acceptance criteria. Note that success metrics indicate the desired business outcomes, while acceptance criteria indicate when the solution is working correctly. If there is no way to measure success, link to an issue that will implement a way to measure this.
Create tracking issue using the the Snowplow event tracking template. See https://gitlab.com/gitlab-org/gitlab/-/blob/master/.gitlab/issue_templates/Snowplow%20event%20tracking.md
-->
### What is the type of buyer?
<!-- What is the buyer persona for this feature? See https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/buyer-persona/
In which enterprise tier should this feature go? See https://about.gitlab.com/handbook/product/pricing/#four-tiers -->
### Is this a cross-stage feature?
<!-- Communicate if this change will affect multiple Stage Groups or product areas. We recommend always start with the assumption that a feature request will have an impact into another Group. Loop in the most relevant PM and Product Designer from that Group to provide strategic support to help align the Group's broader plan and vision, as well as to avoid UX and technical debt. https://about.gitlab.com/handbook/product/#cross-stage-features -->
### Links / references
<!-- Label reminders - you should have one of each of the following labels if you can figure out the correct ones -->
issue