make use of global agent

[climba03003]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

It should be a good feature to use the global dispatcher of undici, http, https if someone provide a custom one.
For example, when sharing a global ProxyAgent between this plugin and http client.

Motivation

No response

Example

Since when we use getGlobalDispatcher, undici will create one when not exist.
We need to explicitly opt-in for the global dispatcher to prevent any strange behavior.

I would assume someone need to use it by.

import { setGlobalDispatcher, ProxyAgent } from 'undici'

const proxyAgent = new ProxyAgent('my.proxy.server')
setGlobalDispatcher(proxyAgent)

fastify.register(FastifyReplyFrom, {
  base: 'http://localhost:3001/',
  // http and https is allowed to use http.globalAgent or https.globalAgent
  globalAgent: true, 
  undici: {
  }
})