nixos/suwayomi-server: add option `settings.server.extensionRepos`

This option is now a key feature of Suwayomi-Server, so it is
interesting to have it directly available in a quick search.

nixos/modules/services/web-apps/suwayomi-server.md

@@ -101,6 +101,9 @@ Not all the configuration options are available directly in this module, but you
         port = 4567;
         autoDownloadNewChapters = false;
         maxSourcesInParallel" = 6;
+        extensionRepos = [
+          "https://raw.githubusercontent.com/MY_ACCOUNT/MY_REPO/repo/index.min.json"
+        ];
       };
     };
   };

nixos/modules/services/web-apps/suwayomi-server.nix

@@ -102,6 +102,17 @@ in
                 '';
               };
 
+              extensionRepos = mkOption {
+                type = types.listOf types.str;
+                default = [];
+                example = [
+                  "https://raw.githubusercontent.com/MY_ACCOUNT/MY_REPO/repo/index.min.json"
+                ];
+                description = mdDoc ''
+                  URL of repositories from which the extensions can be installed.
+                '';
+              };
+
               localSourcePath = mkOption {
                 type = types.path;
                 default = cfg.dataDir;