[GitLab] Add support for confidential issues

[tulir]

Confidential issues have the same payload as normal issues, but they have a separate hook type. This pull request adds support for the Confidential Issue Hook event type.

[deankarn]

Thanks @tulir

I would prefer to do it a slightly different way so that the end user can distinguish between Issues and confidential issues when the payload comes through by:

• having a separate confidential payload type (can embed the issue type) and just in case they types ever diverge it's easy to separate them without making breaking changes.