Refactor config

[dworthen]

• Add new ModelConfig to represent LLM settings
  • Combines LLMParameters, ParallelizationParameters, encoding_model, and async_mode
• Add top level models config that is a list of available LLM ModelConfigs
• Remove LLMConfig inheritance and delete LLMConfig
  • Replace the inheritance with a model_id reference to the ModelConfig listed in the top level models config
• Remove all fallbacks and hydration logic from create_graphrag_config
  • This removes the automatic env variable overrides
• Support env variables within config files using Templating
  • This requires "$" to be escaped with extra "$" so ".\.txt$" becomes ".\.txt$$"
• Update init content to initialize new config file with the ModelConfig structure

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

Additional Notes

[Add any additional notes or context that may be helpful for the reviewer(s).]