Ship prebuilt libpoplar_julia as a JLL

[giordano]

We should be able to build the C++ wrapper around libpoplar as a JLL built on GitHub Actions using Poplar Docker images to obtain the SDK. We will have to augment the platform obtaining the current poplar version from popc --version, and maybe a preference as an escape hatch to choose a different SDK version and/or library path in case someone wants to test a local build of the library. So we should also make it somewhat easy to still be able to build the library locally.

This should sensibly simplify installation process for most users, avoiding the long wait for building the wrapper.