Resolve "Add support for subgroup events in Group webhooks"

What does this MR do?

For #260419 (closed)

• Adds ability to setup webhooks for subgroup events in /groups/group_name/-/hooks page.
• Adds ability to setup webhooks for subgroup events via the /hooks API
• Supports firing this webhook when a new subgroup is added to this group.
• Supports firing this webhook when a subgroup is removed from this group.

Support for firing this webhook when a subgroup is updated is planned for later and is not part of this MR.

Screenshots (strongly suggested)

Setting up webhook

Webhook data when a subgroup is added

Webhook data when a subgroup is removed

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #260419 (closed)