Add support for Anthropic API

[annanay25]

Recently we've seen great performance from the Claude model family and it would be nice to add support for these models to the LLM app.

API reference: https://docs.anthropic.com/en/api/messages
Typescript SDK: https://github.com/anthropics/anthropic-sdk-typescript

[csmarchbanks]

Let's start with a spike for determining pros/cons of doing this in openrouter vs implementing directly in the plugin backend.

[sd2k]

For doing it directly: ideally we'd be able to find something like the litellm Python library which lets you call tons of LLM providers using an OpenAI-like request/response format, but I can't seem to find one for Go; we'd probably need to re-implement the Anthropic part of litellm ourselves and keep it maintained.

[ishaan-jaff]

Hi @sd2k thanks for considering litellm (I'm one of the litellm maintainers) . If you'd like you can also use our proxy server https://docs.litellm.ai/docs/proxy/docker_quick_start and just point to it

That way you don't need to maintain code in GO