Provide an option to link to a devcontainer definition in a repo

It would be great if users could add a devcontainer.json file that actually just links to a devcontainer somewhere else. Say a user could add this devcontainer.json to their repo:

{
  "devcontainer": "julia"
}

And then the clients would pull the devcontainer file that is defined in this repository here.

The main benefit would be that if we make an update to the Julia devcontainer template, users would automatically get these updates and wouldn't have to update the devcontainer.json files in the repository.

For example, I've just opened a PR for a Julia devcontainer here. It is nice, but I'm sure we'll want to do more fancy stuff in future iterations in the postCreateCommand element. With my proposal in this issue here, any users who adds this to their repository today would automatically get these benefits down the road.