feat: allow to custom manifest JSON

[chenjiahan]

Summary

Allow to custom manifest JSON via output.manifest.generate function.

Documentation will be added in the next PR.

Related Links

• feat(plugin-react-router): React Router 7 / Remix support for Rsbuild #4186 (comment)
• https://www.npmjs.com/package/webpack-manifest-plugin#generate

Checklist

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

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