feat: harden fetch route handling and add .all() method

[p-dubovitsky]

Summary

This PR tightens @fastify/fetch route behavior, expands the routing API with fastify.fetch.all(), and improves package readiness for publishing.

Changes

• Isolate raw body parsing to fastify.fetch.* routes so regular Fastify routes keep their normal content-type parsers.
• Stream Response.body to replies instead of buffering the whole response.
• Build Request.url from Fastify request.protocol, request.hostname, and request.url, including trustProxy behavior.
• Add fastify.fetch.all(path, handler).
• Add TypeScript support and type tests for fetch.all().
• Add regression tests for parser isolation, streaming bodies, forwarded protocol URLs, and fetch.all().
• Add package metadata hardening with engines.node and a publish files whitelist.
• Update README for the new API.

Checklist

• run npm run test && npm run benchmark --if-present
• 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