nixos/roundcube: add roundcube module

[Vskilet]

Motivation for this change

I added Roundcube package (#47270) one week ago, so it sound good to have the module to use it 👍
It's my first pull request for a nixos module, so feedbacks are welcome.

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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[globin]

See inline comments and also I'd be happy if you could add a test for it!