feat: adding support for other LLM providers with litellm

[krrishdholakia]

Hi @ErikBjare,

Noticed you're calling GPT-4 + local models via llama cpp. Wanted to see if we could help with LiteLLM (https://github.com/BerriAI/litellm).

Replaced the openai chatcompletions call with litellm.completion.

This should enable gptme to support Anthropic, Bedrock, AI21, TogetherAI, Azure, etc.

Curious - i know you've made a PR into LiteLLM before. What was litellm missing to be useful to you? Any feedback here would be helpful.