nixos/caddy: optional formatting of Caddyfile

As explained in
caddyserver/caddy#5930 (comment),
`caddy fmt` doesn’t handle heredocs. The existing module always runs
`caddy fmt` on native builds, and thus for a configuration containing a
heredoc, the service fails at runtime.

This change is simple, and backwards compatible. Alternatives I
considered include only using the output of `caddy fmt` if `caddy adapt`
also succeeds, removing the formatting and adding an option defaulting to false.
Adding an option that defaults to true allows users to disable `caddy fmt`
for other reasons, thoguh.

nixos/doc/manual/release-notes/rl-2311.section.md

@@ -511,6 +511,8 @@ The module update takes care of the new config syntax and the data itself (user
 
 - `services.bitcoind` now properly respects the `enable` option.
 
+- The Caddy module gained a new option named `services.caddy.formatCaddyfile` which is enabled by default, to match existing behaviour. When disabled, `caddy fmt` will no longer run on the generated caddyfile. This is useful where the caddyfile is not supported by `caddy fmt`, for example [for heredocs](https://github.com/caddyserver/caddy/issues/5930).
+
 ## Nixpkgs internals {#sec-release-23.11-nixpkgs-internals}
 
 - The use of `sourceRoot = "source";`, `sourceRoot = "source/subdir";`, and similar lines in package derivations using the default `unpackPhase` is deprecated as it requires `unpackPhase` to always produce a directory named "source". Use `sourceRoot = src.name`, `sourceRoot = "${src.name}/subdir";`, or `setSourceRoot = "sourceRoot=$(echo */subdir)";` or similar instead.

nixos/modules/services/web-servers/caddy/default.nix

@@ -45,7 +45,8 @@ let
           caddy fmt --overwrite $out/Caddyfile
         '';
       in
-      "${if pkgs.stdenv.buildPlatform == pkgs.stdenv.hostPlatform then Caddyfile-formatted else Caddyfile}/Caddyfile";
+      "${if cfg.formatCaddyfile && pkgs.stdenv.buildPlatform == pkgs.stdenv.hostPlatform
+         then Caddyfile-formatted else Caddyfile}/Caddyfile";
 
   etcConfigFile = "caddy/caddy_config";
 
@@ -149,6 +150,20 @@ in
       '';
     };
 
+    formatCaddyfile = mkOption {
+      default = true;
+      type = types.bool;
+      description = lib.mdDoc ''
+        Format the generated caddyfile using `caddy fmt` if `caddy` is available
+        (i.e. if buildPlatform = hostPlatform) (defaults to true to match behaviour
+        in earlier versions of this module).
+
+        Caddy fmt does not handle all valid Caddyfiles. For example, see
+        <https://github.com/caddyserver/caddy/issues/5930> for the lack of heredoc
+        support.
+      '';
+    };
+
     configFile = mkOption {
       type = types.path;
       default = configFile;