Add Integrations Menu Item and List to Group Settings

What does this MR do?

Issue #209823 (closed)

• Add menu item Settings > Integrations to Group owners.
• Add a listing of available Integrations on the group level.
• Behind a feature flag group_level_integrations scoped to each group. Feature flag is used mainly to wait until the backend is complete so can be rolled out properly. Can be accessed locally at http://localhost:3000/groups/h5bp/-/settings/integrations.

Screenshots

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