nixos/avahi: add support for extraConfig

[ljani]

Motivation for this change

Allow setting additional options for avahi-daemon.conf.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Fits CONTRIBUTING.md.

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[infinisil]

Can you change the commit message to use the "nixos/avahi" prefix? Looks good to me :)

[ljani]

@infinisil Done. Thanks for the clear up, I was indeed a little confused by the module and pkg-name separation when I took a peek what the other committers had done.