Handle creating confidential draft comment in MR
## Description Currently the BE and FE do not support creating a confidential draft comment in Merge Requests. ## Context The following discussion from !51718 should be addressed: - [ ] @leetickett started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51718#note_516359568): (+2 comments) > TODO: Handle confidential comments??? /cc @leetickett @pedroms
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