Add rudimentary Octave interface

[redstone99]

This adds a basic Octave interface that exposes a function piqp(), that invokes the dense PIQP solver and returns the result. The integration mainly translates between Eigen3 data structures and Octave native data structs and then passes args & results back & forth between the C PIQP implementation and Octave.

This integration is not very polished and the CMake support is pretty basic.
I played around with it on Ubuntu 24.04 and Octave 8.4.0

If it looks ok, I think there's a way to merge and squash all the commits into one commit, but let me know if not, and I'll squash things on my end.

[RSchwan]

In the end, I decided to port the Matlab interface to Octave and added proper packaging (see .github/workflows/octave.yml). There are still some open points before I merge this PR which I try to address in the coming days:

• The Windows build doesn't seem to work yet, i.e., the CI looks like it's not working
• The documentation has to be updated

You can also directly download the zipped package here: https://github.com/PREDICT-EPFL/piqp/actions/runs/9497107452 if you want to give it a test.

[redstone99]

Wow, nice work - looks like you did a much more comprehensive job of adapting to Octave than the bare-bones pull request I did.
I downloaded the artifact, started octave and ran "pre_install" and it completed without error.
It might be helpful to have pre_install print out where it dumped output files (./inst) and maybe some options for next steps or link to documentation of next step.

Shall I close my pull request?