feat: add asyncLocalStorage option for external AsyncLocalStorage inj…

[katreniak]

…ection

Add an asyncLocalStorage plugin option to allow injecting an external AsyncLocalStorage instance. This enables sharing a single AsyncLocalStorage across multiple request sources in applications that handle requests from various origins.

Use cases:

• Fastify HTTP requests
• Queue consumers
• Scheduled tasks and timers
• Other HTTP servers or frameworks

Previously, the plugin created its own AsyncLocalStorage instance, making it difficult to maintain a unified request context across different entry points. By providing the asyncLocalStorage option, users can now create a central AsyncLocalStorage instance and share it across all request handlers, without coupling non-HTTP code to Fastify.

Usage:

  const sharedALS = new AsyncLocalStorage()
  app.register(fastifyRequestContext, {
    asyncLocalStorage: sharedALS
  })

Breaking change: None.

• The plugin maintains backward compatibility by using a default AsyncLocalStorage instance when the setter is not called.
• Exported asyncLocalStorage is untouched.

Checklist

• [x] run npm run test && npm run benchmark --if-present
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct