API endpoint for importing a project export

The GitLab project export is useful for backing up individual projects and transferring projects between instances. An API is being added to allow exports to be automated. We should add a corresponding API for importing project exports.

Proposal

Extend POST /projects with support for creating a project from a project export

Links

https://gitlab.com/gitlab-org/gitlab-ce/issues/29130

https://docs.gitlab.com/ce/user/project/settings/import_export.html#importing-the-project