random content range recipe

[boonious]

This PR introduces a recipe type that enables OriginSimulator to service response payloads of random and varying sizes within a given range, e.g. between 300kb and 400kb in the recipe below.

It also adds a Simulation data struct, and a separate GenServer that injects flakiness (varying response payload sizes) into simulation without incurring performance cost on the main simulation server.

{
    "route": "/*",
    "random_content": "300kb..400kb",
    "stages": [
        { "at": 0, "status": 200, "latency": 0}
    ],
    "headers": {
      "content-encoding": "gzip"
    }
}