add add-hoc rerank implementation to embedding, add async rerank

[emrgnt-cmplxty]

[!IMPORTANT] Adds asynchronous reranking to embedding providers and updates reranking configuration and dependencies.

• Behavior:
  • Adds arerank() method to EmbeddingProvider class in embedding.py for asynchronous reranking.
  • Implements arerank() in litellm.py, ollama.py, and openai.py to return reranked results or original results if reranking fails.
  • Updates rerank() in litellm.py to support reranking via HuggingFace API.
• Configuration:
  • Replaces rerank_dimension and rerank_transformer_type with rerank_url in EmbeddingConfig.
  • Updates r2r.toml to include rerank model configuration.
• Dependencies:
  • Adds aiohttp to pyproject.toml for asynchronous HTTP requests.

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