Add support for custom download URL in a similar way as Rclone's: https://rclone.org/b2/#b2-download-url

Custom endpoint for downloads.

This is usually set to a Cloudflare CDN URL as Backblaze offers free egress for data downloaded through the Cloudflare network. Rclone works with private buckets by sending an "Authorization" header. If the custom endpoint rewrites the requests for authentication, e.g., in Cloudflare Workers, this header needs to be handled properly. Leave blank if you want to use the endpoint provided by Backblaze.