Artifact Cache Sharing

This adds initial support for artifact sharing with HTTP, SSH and local repositories. No push support is available for HTTP repositories. SSH artifact sharing requires sshfs.

It is planned to support querying the remote repository for available artifacts before running the build pipeline in the future. This will allow an optimized pipeline, completely skipping build dependencies of already built elements. This will also allow us to again remove the Element.__built state as we can simply push all artifacts that are not available on the remote repository.