Feat/ balancedPool add

[gulbaki]

• Detect Array.isArray(opts.base) && opts.base.length > 1
  instantiate new undici.BalancedPool(origins, undiciOpts).
• Keep existing single-upstream behaviour untouched.
• Added one unit tests covering balanced.
• Updated docs
  fixes: Support load balancing across multiple base urls. #208

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

[mcollina]

lgtm

[gurgunday]

code LGTM

[Antiavanti]

@gulbaki I think this is not compatible with #157.
Or at least getUpstreamcannot be used when base: string[] is used (which should be documented).

Even type in https://github.com/fastify/fastify-reply-from/pull/157/files#diff-7aa4473ede4abd9ec099e87fec67fd57afafaf39e05d493ab4533acc38547eb8R54 is not adjusted.
base: string where in fact is now base: string | string[].