add Cohere Client to autogen_ext

[hashi-hiroy]

Why are these changes needed?

I implemented the client required to access the models in cohere SaaS.

Related issue number

Checks

• [x] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• [x] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [x] I've made sure all auto checks have passed.

[hashi-hiroy]

@microsoft-github-policy-service agree

[priteesshhh]

I analyzed this issue but couldn't identify the affected files. Could you please provide more details about which files need to be modified?