anthropicllm: Allow env var ANTHROPIC_MODEL for setting Claude model

[aguynamedben]

There was no way to set the Anthropic model via env var, this adds that.

I made model lowercase in the Anthropic options because I was following the patterns in the Cohere LLM and there's no need for it to be public.

Demo

Slower with Opus set via env var

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Faster with Haiku set via env var

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404 as expected with invalid value sent to Anthropic

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