Add support for AbortSignal to enable request cancellation

[drummer1992]

Changes:

• Add abortSignal property to Request class (initialized as null)
• Implement setAbortSignal(abortSignal) method for fluent API
• Pass abort signal through the request pipeline to underlying HTTP clients
• Update sendNodeAPIRequest to accept and apply abort signal to fetch options
• Update sendXmlHttpRequest to accept and apply abort signal to XMLHttpRequest
• Add test coverage for abort signal functionality

Usage:

const abortController = new AbortController();

Request.get('https://api.example.com/data')
  .setAbortSignal(abortController.signal)
  .then(result => console.log(result));

// Cancel the request
abortController.abort();