Add the ability to define rack-attack rules via a JSON file

[pnomolos]

The company I work for would like to be able to define rack-attack rules via a JSON format to make them simpler to test, easier to verify, and introduce the possibility that most of our attack rules could be pushed to a dedicated WAF while also providing developers with the same experience in their development environments.

I've put together a branch that's modelled after Cloudflare's Ruleset Engine. Because I was feeling ambitious, I also wrote a Rust-based extension to handle evaluation of any rules defined via the JSON format. We see about 1.5 billion requests per week, so at our scale those sorts of micro-optimizations make sense (at least until we can push rules to a WAF directly).

Is this something that interests the rack-attack team? I'd be more than happy to pull the branch into a few disparate PRs, for example if you'd like the JSON format but not the Rust extension.

The code is available here. Documentation for the JSON format is specifically available here.