Support a new LLM Provider

[sestinj]

Continue supports many different LLM providers by subclassing the BaseLLM class. If you know of an LLM provider that we don't support, adding it can be almost as simple as writing a single method. See CONTRIBUTING.md for a full walkthrough on adding a provider.

Some providers we don't yet support:

• Cohere
• Amazon Bedrock

[KingMob]

Would be great to support gpt4all, since they seem to have the best local model set up these days.

[bjornjorgensen]

what about adding https://github.com/BerriAI/litellm ?