Abstract Publisher APIs from the router

1. Move fetchMetadata and serve static files to local publisher
2. Create factory class to create publishers based on app cofig

packages/backend/src/plugins/techdocs.ts

@@ -49,7 +49,7 @@ export default async function createPlugin({
   const urlPreparer = new UrlPreparer(reader, logger);
   preparers.register('url', urlPreparer);
 
-  const publisher = new Publisher(logger, config, discovery);
+  const publisher = Publisher.fromConfig(config, logger, discovery);
 
   const dockerClient = new Docker();