Add MulticlassJudge detector for configurable LLM-as-judge classification

[ABeltramo]

Introduces a new MulticlassJudge detector that extends ModelAsJudge with JSON-aware response parsing and user-defined classification categories (e.g. complied/rejected/alternative/other). Supports configurable system and user prompts, custom score keys/fields, confidence thresholds, and optional JSON schema injection for structured output APIs.

Cherry-picked from trustyai-explainability/garak:automated-red-teaming

[ABeltramo]

From a quick glance it seems that the CI failures are unrelated to this PR.
The only failing test is tests/generators/test_litellm.py::test_litellm_model_detection, which fails with:

  openai.OpenAIError: Missing credentials. Please pass an `api_key` ...

This test has no OPENAI_API_KEY skip guard (unlike the other tests in the same file), so it runs unconditionally whenever litellm is installed. The same test passes on our fork's CI, which ran against a slightly older package version, so I guess this is just a flaky test..

[jmartin-tech]

The test failure is from an environment requirements change in the litellm dependency released yesterday, a PR to address it in main should be up by end of day a propagate it to the feature branch.