Add the GitLab Service Desk information and controls to the Projects API

Problem to solve

The Projects API doesn't have any information about GitLab Service Desk: whether or not it is turned on and what the email address is if it is on.

Intended users

None of these fit exactly, but the closest would be:

Further details

Any time I use the GitLab API to retrieve information about my projects, I should be able to see if Service Desk is turned on and what the email address is.

I should also be able to turn Service Desk on or off from an API call.

Permissions and Security

I assume the permissions should be in sync with otherwise viewing/editing project information through the API.

Documentation

Should be added to https://docs.gitlab.com/ee/api/projects.html in all applicable places.

Testing

Requires API testing and possibly confirmation via the UI.

What does success look like, and how can we measure that?

API users can successfully view and edit Service Desk information.

What is the type of buyer?

Whoever buys Silver/Premium to get Service Desk would likely appreciate API access to the feature.