Feat/cohere agent implementation

[17ColinMiPerry]

Pull Request Type

• ✨ feat
• 🐛 fix
• ♻️ refactor
• 💄 style
• 🔨 chore
• 📝 docs

Relevant Issues

resolves #4490

What is in this change?

Adds @agent support for Cohere. Uses v1 SDK to be consistent with the LLM provider implementation.

#convertChatHistoryCohere: converts standard messages to the format expected by Cohere.
#handleFunctionCallStream/Chat: Complies to Cohere API by passing message + chat_history
streamingFunctionCall: Overridden from UnTooled to handle Cohere's stream event format
stream/complete: Overrides to handle Cohere's SDK response structure

Additional Information

Currently, we don't have the @langchain/cohere package, so there is no LangChain integration.
https://docs.cohere.com/v1/reference/about

Developer Validations

• I ran yarn lint from the root of the repo & committed changes
• Relevant documentation has been updated
• I have tested my code functionality
• Docker build succeeds locally

[17ColinMiPerry]

The v1 SDK was used to be consistent with the rest of the repository. However, it seems that v1 is largely being deprecated, and v2 is OpenAI-like. Would it be meaningful to convert this part of the codebase (and the Cohere LLM provider) to the v2 SDK API at this time?