feat: make LiteLLM dependency optional #967

Feature description

LiteLLM should be removed from default paths.
OpenAI / Anthropic / Google Gemini dedicated classes should be created, with OpenAI as the default option

• dependency on LiteLLM can stil exist, ~ 80% should work without it
• rerankers (ragbits-document-search) can be left as-is
• add new LLM classes
• add new embedder classes
• core functionality:
  • focus on supporting OpenAI / Anthropic / Google Gemini
  • support Ollama / VLLM (hosted locally)

Motivation

LiteLLM should not be a mandatory dependency

Additional context

No response