CohereChatGenerator should list supported models #2915
Description
Similar to the SUPPORTED_MODELS class variable that we added to the OpenAIChatGenerator in PR deepset-ai/haystack#10736, the CohereChatGenerator should list supported models in a SUPPORTED_MODELS class variable, the docstring should link to the model provider's web page that lists models and there should be a unit test that checks that SUPPORTED_MODELS is a non-empty list of strings. See deepset-ai/haystack#10736 for an example.