feat: support async function for rsbuildConfig in createRsbuild options

[chenjiahan]

Summary

This is one of the prerequisites for implementing the server.restart method.

To allow the loadConfig method to be called again when restarting the server, we need to allow createRsbuild to call loadConfig. Therefore, this PR supports creatingRsbuild's rsbuildConfig to pass in an async function to dynamically load the configuration.

import { createRsbuild, loadConfig } from '@rsbuild/core';

const rsbuild = await createRsbuild({
  rsbuildConfig: async () => {
    const { content } = await loadConfig();
    someFunctionToUpdateConfig(content);
    return content;
  },
});

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

[netlify]

✅ Deploy Preview for rsbuild ready!

Name	Link
🔨 Latest commit	a4e76ee
🔍 Latest deploy log	https://app.netlify.com/sites/rsbuild/deploys/67d2b9eef1a78f0008dbe060
😎 Deploy Preview	https://deploy-preview-4778--rsbuild.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 54 (🔴 down 21 from production) Accessibility: 97 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: 60 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.