Utils for gacha history record subsystem

* Auto generate mapping files with command `java -jar grasscutter.jar -gachamap`

* Static file provider
  * For gacha record webpage
  * All static files should be stored at `GRASSCUTTER_RESOURCE/gcstatic/`
  * Can benefit other subsystem in future when webpages involved

src/main/java/emu/grasscutter/server/http/gcstatic/StaticFileHandler.java

@@ -0,0 +1,31 @@
+package emu.grasscutter.server.http.gcstatic;
+
+import java.io.File;
+import java.io.IOException;
+
+import emu.grasscutter.Grasscutter;
+import express.http.HttpContextHandler;
+import express.http.Request;
+import express.http.Response;
+
+public final class StaticFileHandler implements HttpContextHandler {
+	String static_folder;
+	public StaticFileHandler() {
+        static_folder = Grasscutter.getConfig().RESOURCE_FOLDER + "/gcstatic";
+	}
+
+	@Override
+	public void handle(Request req, Response res) throws IOException {
+        // Grasscutter.getLogger().info( req.path());
+
+        String reqFilename = req.path().replace("/gcstatic", ""); // remove the leading path
+        reqFilename = reqFilename.replace("/../", "/./"); // security guard to prevent arbitrary read
+        File resFile = new File(static_folder + reqFilename);
+        if (resFile.exists()) {
+            res.sendFile(resFile.toPath());
+        } else {
+            res.status(404);
+            res.send("404");
+        }
+    }
+}