Idea: CMake `fetch_content()` if no `Rack_DIR`

[nathanjhood]

Chore

Describe the chore

Use CMake's fetch_content_and_declare() - or whatever the correct sig is - to acquire Rack SDK, in the case that no RACK_DIR was either found or passed in.

Additional context

Haven't any experience with this particular CMake function so far, but I've already written the vcpkg portfile to give a good working idea of something I assume is very similar.

I'm not sure where the DL'd content goes... hopefully into the binary dir? Hmm.