HTTP-layer toxics

[cbgbt]

Is your feature request related to a problem? Please describe.
In my integration tests, I'd like to produce chaos at the TCP layer, but also at the HTTP layer.

Describe the solution you'd like
I'm wondering how you would feel about introducing the concept of HTTP toxics, possibly under a feature flag. I haven't looked too closely at your architecture/implementation yet, but possibly these could consume the entire input stream to a buffer and attempt to parse them as an http::Request.

Describe alternatives you've considered
To solve right now this I've written a very small axum proxy server with middleware that randomly returns early with 503 errors, and we chain this with noxious to produce the desired result.

Additional context
This idea may be a nonstarter, since I think it doesn't play very nicely at all with the directionality approach currently present.

Seems like there's a similar issue in the toxiproxy repo: Shopify/toxiproxy#50