Handle uBO style polyfills for blocked requests

[pes10k]

uBO defines polyfills, or alternate content bodies that should be returned for resources from a URL that matches a pattern. The bodies of these responses, and identifiers for them, are given here:

https://github.com/uBlockOrigin/uAssets/blob/master/filters/resources.txt

Rules then reference these polyfils with the redirect=<identifier> option on rules.

It would be good if adblock-rust both could

1. understand the redirect option in rules
2. parse polyfil definitions like https://github.com/uBlockOrigin/uAssets/blob/master/filters/resources.txt
3. and return the alternate message body when appropriate