Support `exec` by `--id-label`

Right now, `--workspace-folder` is a mandatory argument to `exec`. This makes sense if the devcontainer was started from the environment that's running the CLI, from a locally-accessible filesystem. However, if the container was started by way of VS Code's "Clone Repository in Container Volume" feature, it's impossible to provide the workspace folder, since it only existed within the ephemeral bootstrap container.

It seems like `--id-label` is intended to be an identification mechanism for containers that doesn't require a local copy of their `devcontainer.json` -- if so, it would make sense to support this mechanism for `exec`.