nixos/nginx: add prependConfig options

This is needed for example to load dynamic nginx modules.
Which need to be loaded before any `http` or `stream` blocks.
See: https://docs.nginx.com/nginx/admin-guide/dynamic-modules/dynamic-modules/#installing-and-loading-the-module

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

@@ -163,6 +163,8 @@ let
   configFile =
     (if cfg.validateConfigFile then pkgs.writers.writeNginxConfig else pkgs.writeText) "nginx.conf"
       ''
+        ${cfg.prependConfig}
+
         pid /run/nginx/nginx.pid;
         error_log ${cfg.logError};
         daemon off;
@@ -835,6 +837,18 @@ in
         '';
       };
 
+      prependConfig = mkOption {
+        type = types.lines;
+        default = "";
+        description = ''
+          Configuration lines prepended to the generated Nginx
+          configuration file. Can for example be used to load modules.
+          {option}`prependConfig` can be specified more than once
+          and its value will be concatenated (contrary to {option}`config`
+          which can be set only once).
+        '';
+      };
+
       appendConfig = mkOption {
         type = types.lines;
         default = "";