Reintroduce custom fetch to buildAuthenticatedFetch as an option

[mrkvon]

Feature description

This PR addresses #3810.

Add the optional fetch parameter back to buildAuthenticatedFetch options (second parameter), defaulting to global fetch. This is a non-breaking change.

Usage:

const authFetch = await buildAuthenticatedFetch(accessToken, {
  fetch: customFetch, // optional
  // ...otherOptions
});

Motivation

Custom fetch was removed in v2 of @inrupt/solid-client-authn-core. But buildAuthenticatedFetch is used externally as well, most notably in automated CSS authentication. While not strictly necessary for most use cases, a custom fetch option gives more flexibility (polyfills, logging, tweaking functionality, etc.) when building authenticated fetches.

Checklist

• All acceptance criteria are met. not found
• Relevant documentation has been written/updated. JSDoc of the affected method
• The changelog has been updated, if applicable. the CHANGELOG doesn't seem to track changes to core; will add upon request
• New functions/types have been exported in index.ts, if applicable. not applicable
• Commits in this PR are minimal and have descriptive commit messages

Fixes #3810