Improve prerequisites for contributors.

[gibbz00]

• Moves the xmake.lua to top level to simplify the generation of a working compile_commands.json.
• Sets up test server certificates and documents their usage.
• Introduces a flake.nix for reproducible build environments, this is then also used in CI.

[gibbz00]

I'm not entirely sure why CI is failing. #6 does it with a nix flake instead, (and ran the pipeline ~2.5x faster). Should I try to fix the Ubuntu one? Or maybe get the nix-based one merged first?

[fantasy-peak]

I'm not entirely sure why CI is failing. #6 does it with a nix flake instead, (and ran the pipeline ~2.5x faster). Should I try to fix the Ubuntu one? Or maybe get the nix-based one merged first?

thanks, I believe using Nix is ok

[gibbz00]

thanks, I believe using Nix is ok

Cool! I moved the commits from that branch into this one.