POC: build a root with a `generate` block that uses `templatefile(path, vars)`

[spilliams]

When I'm writing a root, I don't want to have to type out the entire provider config or backend config in text every time.
Maybe I want to keep that template in its own file, and reference it using terraform's templatefile function.

This is probably where I want to take the old feature idea of "partials".

[spilliams]

This could provide a great example for how users can quickly manage a remote_state data source. See also: #31