RPM API skeleton

Proposal

Add an initial set of grape APIs behind a development feature flag scoped to projects for RPM.

Endpoints needed

1. GET api/v4/project/<project_id>/packages/rpm/<package_file_id>/<filename> - Download RPM package files
2. GET api/v4/project/<project_id>/packages/rpm/repodata/<filename> - Download repository metadata files
3. POST api/v4/project/<project_id>/packages/rpm/<filename> - Upload RPM package files
4. POST api/v4/project/<project_id>/packages/rpm/<filename>/authorize - workhorse accelerated upload authorization. Related to #370869 (closed).

In this issue we are not concerned with functionality, but with defining the endpoints and adding the feature flag scoped to projects for use throughout the development of RPM. All endpoints should return 404.