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
issue