nixos/rspamd: Allow worker type to be proxy again

[griff]

Motivation for this change

When reworking the rspamd workers I disallowed proxy as a type and
instead used rspamd_proxy which is the correct name for that worker
type. That change breaks peoples existing config and so I have made this
commit which allows proxy as a worker type again but makes it behave
as rspamd_proxy and prints a warning if you use it.

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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[infinisil]

LGTM. Eventually I'd like to implement a warning and assertion system into the module system directly, so it should be easier to issue warnings from submodules.

[griff]

@infinisil if you need help building that I am willing.

[dotlambda]

How about using apply in the option declaration instead?

[infinisil]

Ohhh nice, would be a lot easier. Would need a builtins.trace in it though and couldn't use the NixOS warnings options, but that's fine by me.

[griff]

I mostly didn't use apply in order to use config.warnings which seems the standard way of handling warnings.

[infinisil]

I'll argue that having so much code just for this is not worth it just for being more conventional. And actually I think that other modules are also not using config.warnings for submodules for the same reason.

[griff]

Commit has now been rewritten to use apply instead of config.warnings