DEV: Extract configs to a yml file and allow local config

[nattsw]

Eval configs were hard-coded in DiscourseAi::Evals::Llm. This PR moves them into a config file.

Evaluators can now also define their own config locally with config/eval-llms.local.yml, and will have the choice to define the api_key attribute, or api_key_env if an env var is preferred.

Example file:

llms:
  discourse_llm:
    display_name: Qwen/Qwen2.5-32B-Instruct
    name: Qwen/Qwen2.5-32B-Instruct
    tokenizer: DiscourseAi::Tokenizer::OpenAiTokenizer
    api_key: <FILL IN KEY HERE>
    provider: open_router
    url: <FILL IN URL HERE>
    max_prompt_tokens: 128000
    vision_enabled: false

[SamSaffron]

nice change 🤗