nixos/corerad: add settings option to supersede configFile

[mdlayher]

Motivation for this change

Compliance with NixOS/rfcs#42 since @marsam added go-toml which made outputting TOML config trivial. I borrowed the pattern here from the Prometheus module:

nixpkgs/nixos/modules/services/monitoring/prometheus/default.nix

Lines 20 to 24 in 0e040d1

	# Pretty-print JSON to a file
	writePrettyJSON = name: x:
	pkgs.runCommandNoCCLocal name {} ''
	echo '${builtins.toJSON x}' | ${pkgs.jq}/bin/jq . > $out
	'';

We also add a new test for HTTP metrics output and set up a monitoring interface which can't easily be tested, but does verify that the various config modes work.

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.

/cc @aanderse @jonringer @infinisil who discussed adding a settings option with me in my original PR: #77252

[mdlayher]

@GrahamcOfBorg test corerad

[mdlayher]

Config looks sane to me with go-toml's output:

[matt@servnerr-3:~/src/nixpkgs]$ cat /nix/store/zybrs1y0pmwys3djwkbcaskpaclzqv7b-corerad.toml

[debug]
  address = "localhost:9430"
  prometheus = true

[[interfaces]]
  monitor = true
  name = "eth0"

[[interfaces]]
  advertise = true
  name = "eth1"

  [[interfaces.prefix]]
    prefix = "::/64"

I did just notice that #89744 uses the tool yj for this. Is there a defacto config conversion tool in the tree? I'm happy to change if necessary.

[mdlayher]

@GrahamcOfBorg test corerad

[infinisil]

Looks good except for the nit below!

[mdlayher]

@GrahamcOfBorg test corerad