Add Dynamic Download of CSPICE Toolkit

[medley56]

Context

This package relies on the CSPICE Toolkit (which includes MKSPK and MSOPCK) and those are currently packaged with the repo but this is not ideal and NAIF recommends against doing this. Ideally we want to be able to dynamically download the correct binaries and statically link them during package installation for use by the package.

Implementation

Via an "experimental feature", Poetry supports installation tasks via a build.py file along with your package. This is similar to a setup.py file but it doesn't have the same scope of responsibility as setup.py, such as requirements and metadata. We can put the installation logic here. My experiments so far indicate that build.py does work, including for publishing packages, but we will probably have to continue to support changes as Poetry gets updated.

Sadly, Poetry is being pretty slow in implementing this as a stable feature. I see signs on their github issues that development is progressing but it's still not documented outside of the issues themselves.

python-poetry/poetry#2740
python-poetry/poetry#8460
python-poetry/poetry#3576
python-poetry/poetry#8460