Feat add support for openai compatible providers

[nkzk]

Adds support for openai compatible llm providers. Resolves #11

Changes

• cmd/login.go login flow now supports specifying provider, and gives different options based on the provider
• Adjusted pkg/chat/chat.go to fetch and use the selected LLM model from configuration.
  • Note: A bit unsure if how I fetch the config here is optimal. Please review.
• pkg/config/config.go added provider, model, and API version as a configurable parameters.
• Implemented provider-specific logic in pkg/config/provider.go for setting up client configurations.
• Updated DefaultValidator in pkg/config/validator.go to support the new provider field. Set a default model deployment and provider if it has not been set in configuration to avoid breaking changes
• docs: Updated README.md with an alternative login example, and updated azure login example (add provider attribute)

Testing:

Tested with OpenAI API and Ollama.

Please verify non-breaking changes to azure openai deployment configs.