[BUG] Support ANTHROPIC_BASE_URL

ANTHROPIC_BASE_URL/ANTHROPIC_API_KEY is not currently supported. Our organization requires routing LLM queries through a llm specific proxy for tracking and monitoring purposes. Can we support this config and skip the login in the cases where its set?

It seems like theres already a way to do this with Vertex and Bedrock, would be nice to do this natively for the claude api.