feat: Support Anthropic for Langgraph

[f4roukb]

Issue: #94

Description: Introduce basic support for Anthropic LLM inference client. Happy to extend this in the future as-needed.

Changes:

• New dependency: Add the langchain-anthropic dependency to the langgraph extra.
• Add a new LLM config.
• Add unit tests for ChatAnthropic conversion between AgentSpec and Langgraph.
• Update docs.
  No breaking changes.

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• PR author: f4roukb
• farouk.boukil@nexthink.com (@f4roukb)

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[dhilloulinoracle]

Thanks @f4roukb for the contribution!

Once you have signed the CLA we can try to merge it.

If that's too awkward for you, we can add a commit in the PR to add the changelog entry where we also thank you for your contribution.

[sonleoracle]

Thank you for your contribution @f4roukb !

[f4roukb]

Thanks all! I'll have a look at the OCA as soon as possible and get back to you.

[f4roukb]

@dhilloulinoracle, please feel free to commit to this branch. Thanks for crediting me.

I can wrap this up as soon as I get feedback from you guys on the open threads.

[f4roukb]

@cesarebernardis, the PR is reviewable now. Thanks!

[cesarebernardis]

LGTM, thank you for the contribution!

[cesarebernardis]

Don't worry about the failures in the tests. As soon as you will have signed the OCA, I will take care of finalizing and merging the PR.