Support tarball package type in Package Registry

Proposal

Support tarballs in GitLab's Package Repository. Why is this needed when you can create tarball artifacts at the project level? Precisely because we need a place to put tarballs outside of all of our internal projects. As far as I can tell there is no simple way to push a tarball artifact from a project into another project or any of the GitLab registries.

How would the registry look? A simple directory structure with tarball files and their collision-resistant hashes. For example:

tarball-registry/namespace
|
`-- file1.tar.gz
|
`-- file1.sha256
|
`-- file2.tar.gz
|
`-- file2.sha256
etc.