Add CRUD for Group Level Integration Settings

What does this MR do?

Issue #209825 (closed).

Similar to !27557 (merged), just that it's on Group-level.

• All this is behind a feature flag group_level_integrations.
• Add routes to edit / update / test an integration at the group level
• Extract controller actions of admin/integrations_controller to a shared concern and use it in groups/settings/integrations_controller
• Extract edit and _form views to shared/integrations and use them across admin and group levels.

Screenshots

Go to http://localhost:3000/groups/h5bp/-/settings/integrations/jira/edit, replacing the group name h5bp and integration name jira where needed.

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