Empty controller action for GroupCoverage report

What does this MR do?

This MR creates a controller with one action and the required permissions for implementing an endpoint to return group-level code coverage reports

The specs only assert the responses codes that we want to send back and the controller only returns a placeholder empty CSV which is not spec'd.

I'd like to merge the controller itself and the permissions first to get a proper review on the new controller itself, the feature flag (defaulted to off), and license check implementations separately from the logic of returning the actual data.

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