Add more model support via litellm

[seehorne]

Is your feature request related to a problem? Please describe.
Currently, through LiteLLM we offer support for Gemini, OpenAI, and Anthropic models, but these are not the only model families that may be beneficial to support in ProgramAT

Describe the solution you'd like

• Find more model families that ProgramAT could support
• Integrate this support into existing codebase
• Update readme with list of newly supported model(s)
• Update readme with instructions for how to procure keys for newly supported model(s)
• Update .env.example with spaces for keys for newly supported model(s)

Additional context
When making a PR to resolve this issue, please make clear which models/model families you have added. It is possible this issue will be re-opened, even if you have completed it successfully, to support more expansion, though in doing so we would want to update the description of what is already supported.

[HirenGajjar]

Hii @seehorne, I would like to work on it!