llms/anthropic: Implement WithBaseURL and WithHTTPClient for Anthropic LLM

[GRVYDEV]

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This PR implements the WithBaseURL and WithHTTPClient options for the anthropic llms and wires them up. This implementation was derived from the llms/openai implementation. This is needed to enable using a tool such as Helicone with the Anthropic models (I suspect more people may start using them thanks to Claude 3).

[tmc]

LGTM

[tmc]

hm, we may want to factor this interface out to be in a more general spot.