Build the libpoplar wrapper in CI

[giordano]

Continuation of #37.

Proof of concept for #18.

Still lots to do:

• the resulting library is only 21 kB, it's definitely wrong, it should be over ~40 MB, investigate what's the problem (resolved: I missed to pass the input file template.cpp to the compiler, so it was building an empty library. Now I have some compilation errors, but it's good, it means it's trying to actually compile the right file)
• fix compilation errors emerging after fixing the above point (was missing -I{includedir}/julia
• link libjulia and libcxxwrap
• augment platform to build different wrappers for each SDK version, and the julia version
• provide method to build wrapper locally
• remove from project of the main package redundant dependencies that are needed only to build the wrapper
• require julia v1.9
• write documentation