Support more artifact providers (GitLab)

[filiptibell]

We should add support for more artifact providers such as GitLab. The ArtifactProvider enum and rest of the artifact code was designed with this in mind:

https://github.com/filiptibell/rokit/blob/6ecef4fdb14911005126f23010464476380368c7/lib/sources/artifact.rs#L19-L28

Any new artifact provider just needs an addition to this enum and a client implementation mirroring the one that already exists for GitHub.