Update CreateTestCase Mutation to Accept Confidential Attribute

As part of &11229 (closed), we will be adding support for creating and editing confidential test cases

While test cases will be migrated to the work items framework mid-next year, this is an interim solution to help enable the Quality team to manage test cases within a single source of truth for ongoing confidential projects.

Proposal

Update the CreateTestCase mutation to accept a confidential attribute. This mutation is currently used by the frontend when creating new test cases in a project. The underlying service used by this mutation is Issues::CreateService, which already supports confidentiality.