Feature Request: Add packages directly from github

[loreto]

It's great to be able to add packages from nixpkgs but some times we want to grab a very specific version of a package that it's only available in a github repository.

Add the ability for devbox to add packages from github.

For example:
devbox add stedolan/jq@1.6 in order to install https://github.com/stedolan/jq/releases/tag/jq-1.6