support JSON config as well as YAML

[leondz]

Summary

YAML isn't a universally powerful and flawless format. Consider also supporting JSON for standard configs.

Basic example

E.g. fast.yaml would become:

{
  "system": {
    "parallel_attempts": 20,
    "lite": true
  },
  "run": {
    "generations": 5
  },
  "plugins": {
    "probe_spec": "continuation,dan,encoding.InjectBase64,encoding.InjectHex,goodside,av_spam_scanning,leakreplay,lmrc,malwaregen.SubFunctions,malwaregen.TopLevel,packagehallucination,realtoxicityprompts.RTPIdentity_Attack,realtoxicityprompts.RTPProfanity,realtoxicityprompts.RTPSexually_Explicit,realtoxicityprompts.RTPThreat,snowball,xss",
    "extended_detectors": false
  }
}

Motivation

Why are we doing this? What use cases does it support? What is the expected outcome?

1. YAML lacks some definitional strength, making certain values difficult to express, and an alternative is useful
2. The internal config structure is agnostic to which text format the config is stored in, hopefully keeping the scope of code changes relatively limited
3. It's good to provide us with flexibility around something as core and powerful as configs
4. JSON schema validation is trivial and built-in to python, enabling strong config checking pre-run
5. We will gain a quick, reasonable answer to the various gripes about yaml that I (and presumably also the world) have, like how the official YAML implementation in Python doesn't follow the official YAML spec