Feature/add public email to users api

What does this MR do?

Adds the attribute public_email to the api endpoints that use the API::Entities::User entity ( deployments, keys, runners, jobs, users).

As this information is exposed already on the user profile it should be safe to expose it in the API as well.

Note: I also added some missing attributes to the API docs.

The development of this MR is sponsored by @ siemens (/cc @bufferoverflow).

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39346

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides