`LangchainChatLlm` support for Anthropic

[mongodben]

Jira: n/a

Changes

• Modify langchain chatllm implementation to support Anthropic models

Notes

• i believe this change is compensating for a bug on langchain's side, as we were already conforming to their interface
• skipping test for anthropic b/c MongoDB doesn't have an Anthropic subscription that we could use in the integration test here.
  • You can sign up for free to anthropic and plug in your own API key (that's what i did)
• Thank you to @eric-gardyn for help with the solution implementation!

Resolves https://github.com/mongodb/chatbot/issues/382