perf: do not mutate req.raw

[gurgunday]

Since we are using a Symbol to mark the request as multipart, I can't see a reason not to do this under fastify.req where we can decorate the object in advance and potentially get a performance improvement

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

[Eomm]

👀

[mcollina]

lgtm