feat: allow onBeforeStartDevServer hook to return post callback

[chenjiahan]

Summary

Allow onBeforeStartDevServer hook to return a post callback, the post callback will be called when the default middlewares are registered:

const plugin: RsbuildPlugin = {
  name: 'test-plugin',
  setup(api) {
    api.onBeforeStartDevServer(async ({ server }) => {
      server.middlewares.use((req, res, next) => {
        if (req.url === '/test.html') {
          res.end('Hello, world!');
        } else {
          next();
        }
      });

      // the post callback will be called when the default middlewares are registered
      return () => {
        server.middlewares.use((req, res, next) => {
          if (req.url === '/test2.html') {
            res.end('Hello, world2!');
          } else {
            next();
          }
        });
      };
    });
  },
};

Documentation will be updated later.

Checklist

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

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