Motivation for this change

Although this entire package is actually included in the asio library, which is already present in Nixpkgs, there is a valid reason for introducing it as a standalone package:

The headers implement a proposed C++ standard for networking. asio in fact provides these interfaces, but namespaced under asio, which leaks implementation details. Until this issue is resolved, we are best off using this generated code as it's unclear how to get asio to export the exact interface from the standard ourselves.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.