feat: add run_async for CohereChatGenerator

[Ryzhtus]

Related Issues

• fixes #1383

Proposed Changes:

• Introduced async support in CohereChatGenerator via a new run_async method
• Added async test coverage and included pytest-asyncio as a test dependency

How did you test it?

I added async versions for tests in test_cohere_chat_generator.py

Notes for the reviewer

I've also test live methods with my own API key, but feel free to test yourself

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.