nixos/nextcloud: add overwriteProtocol option

The overwriteprotocol option can be used to force Nextcloud to generate
URLs with the given protocol. This is useful for instances behind
reverse proxies that serve Nextcloud with HTTPS.

In this case Nextcloud can't determine the proper protocol and it needs
to be configured manually.

nixos/modules/services/web-apps/nextcloud.nix

@@ -223,6 +223,19 @@ in {
           <literal>services.nextcloud.hostname</literal> here.
         '';
       };
+
+      overwriteProtocol = mkOption {
+        type = types.nullOr (types.enum [ "http" "https" ]);
+        default = null;
+        example = "https";
+
+        description = ''
+          Force Nextcloud to always use HTTPS i.e. for link generation. Nextcloud
+          uses the currently used protocol by default, but when behind a reverse-proxy,
+          it may use <literal>http</literal> for everything although Nextcloud
+          may be served via HTTPS.
+        '';
+      };
     };
 
     caching = {
@@ -287,6 +300,7 @@ in {
               ${optionalString cfg.caching.apcu "'memcache.local' => '\\OC\\Memcache\\APCu',"}
               'log_type' => 'syslog',
               'log_level' => '${builtins.toString cfg.logLevel}',
+              ${optionalString (cfg.config.overwriteProtocol != null) "'overwriteprotocol' => '${cfg.config.overwriteProtocol}',"}
             ];
           '';
           occInstallCmd = let