feat: support `FormData`

[climba03003]

Support FormData and provide the Content-Type header automatically.

Closes https://github.com/fastify/fastify-multipart/issues/516 Closes https://github.com/fastify/help/issues/1017

Checklist

• [ ] run npm run test and npm run benchmark
• [ ] tests and/or benchmarks are included
• [ ] documentation is changed or added
• [ ] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[climba03003]

The coverage is really broken in supporting FormData. It can only be used in node >= 18 but we still supporting 14 <= node < 18.

That also force me to ignore the coverage for whole new feature. I have tested, when you remove the ignore in node >= 18, it will works.