Support AbortController for aborting fetch operations

[stefan-guggisberg]

Is your feature request related to a problem? Please describe.
AbortController is part of the DOM Living standard. It allows to abort an in-progress fetch operation in a standard way via the signal option. Specifically it allows to apply a timeout to a fetch operation in a standard-compliant portable way.

Describe the solution you'd like
Export AbortController, AbortError and optionally a small helper timeoutSignal for specifying a timeout signal to support the following code snippets:

const { fetch, AbortController, AbortError } = require('@adobe/helix-fetch`);

const controller = new AbortController();
setTimeout(() => controller.abort(), 1000);

try {
  await fetch('https://httpbin.org/delay/2', { signal: controller.signal });
} catch (err) {
  if (err instanceof AbortError) {
    console.log('fetch timed out');
  }
}

or shorter:

const { fetch, timeoutSignal, AbortError } = require('@adobe/helix-fetch`);

try {
  await fetch('https://httpbin.org/delay/2', { signal: timeoutSignal(1000) });
} catch (err) {
  if (err instanceof AbortError) {
    console.log('fetch timed out');
  }
}

The existing non-standard timeout option and TimeoutError should be deprecated.

Additional context
Abortable fetch
fetch timeout discussion

[stefan-guggisberg]

implemented in bb06b3e