Allow upstream to be undici.BalancedPool, pass maxRetriesOn503 to replyOpts

[Antiavanti]

Change upstream type to be compatible with base, based on assumption from fromOpts.base = opts.upstream -> base: string[] introduced in fastify/fastify-reply-from#421 (#279 (comment)).
Pass maxRetriesOn503 to replyOpts, which is not part of FastifyReplyFromOptions, but is expected to be on this level (#34 (comment)).

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

[mcollina]

Can you add a test?