Support files as project[/group/instance] variables
A lot of CI scripts echo the contents of a file, possibly through a base64 decoder, and dump it into a file to be consumed by some other command. It would seem cleaner if we could treat files as first-class citizens. We already do this for service integrations. Why not for the project variables UI? At the very least, this would remove the need to encode/decode the files, which also means we don't need the base64
binary in whatever Docker image is using the file.
Ideally we'd be able to specify a proper filename for the "variable", perhaps even including a path.