add spack-based package file in new spack subdirectory, to allow us to begin maintaining it

[edwardhartnett]

Until now, git was used directly when installing the ufs-weather-model, after dependencies are installed (with hpc-stack or spack-stack).

However, the future is that spack will also be used to install ufs-weather-model, and NCO will not use a git clone command, but will instead use spack to install everything, model and dependencies.

The way we have handled this with other projects has been to add a spack subdirectory at the top level, and to put the package file in there. This version of the package file can then be used for testing, especially testing new capabilities before release. The spack-stack project and main spack repos are updated periodically from this package file.

In order to support this we need to add the package file and some CI files. This will be done by @AlexanderRichert-NOAA .