nginx module: Add extraConfig for locations

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

···
     location ${location} {
       ${optionalString (config.proxyPass != null) "proxy_pass ${config.proxyPass};"}
       ${optionalString (config.root != null) "root ${config.root};"}
+      ${config.extraConfig}
     }
   '') locations);
 in

nixos/modules/services/web-servers/nginx/location-options.nix

···
         Root directory for requests.
       '';
     };
+
+    extraConfig = mkOption {
+      type = types.lines;
+      default = "";
+      description = ''
+        These lines go to the end of the location verbatim.
+      '';
+    };
   };
 }