better API for generating artifact from tarball URL

[simeonschaub]

It would be nice to have a function like

add_artifact!(artifact_toml::String, name::String, tarball_url::String; platform=nothing, lazy=false)

in Pkg.Artifacts, which would automatically download a tarball from url, generate all the needed hashes, and add an artifact with the given name to artifact_toml, optionally specifying a specific platform and whether the artifact should be lazy. BinaryBuilder already does all this, but I think it would be a pretty nice QOL improvement for shipping artifacts independent of BinaryBuilder.

[KristofferC]

https://github.com/simeonschaub/ArtifactUtils.jl solves this imo.