Update Cohere to new endpoint

[flexchar]

What Happened?

I've just learned that Cohere deprecated their generate endpoint and that my fine-tune model is not working with Portkey.

Ref: https://docs.cohere.com/reference/chat

How should we go about this? I am also open to make a PR but if there are some that are using, this may cause a breaking change. A simple major version bump would do?

What Should Have Happened?

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Relevant Code Snippet

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