feat: expose dev server instance for onBeforeStartDevServer hook

[chenjiahan]

Summary

Expose the dev server instance for the onBeforeStartDevServer hook, this makes it easier for the Rsbuild plugins to add a custom middleware.

const myPlugin = {
  setup(api) {
    api.onBeforeStartDevServer(({ server, environments }) => {
      console.log('before starting dev server.');
      console.log('the server is ', server);
      console.log('the environments contexts are: ', environments);
    });
  },
};

const myPlugin = {
  name: 'test-plugin',
  setup(api) {
    api.onBeforeStartDevServer({ server }) => {
      server.middlewares.use((_req, _res, next) => {
        count++;
        next();
      });
    });
  },
};

Checklist

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

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