Patch release v1.4.3 - Make cv2 install optional

As per discussion: https://github.com/vllm-project/vllm/issues/8650 make cv2 optional.

What's Changed

• [Error Message] Improve error message by @patrickvonplaten in https://github.com/mistralai/mistral-common/pull/54
• Make cv2 optional by @patrickvonplaten in https://github.com/mistralai/mistral-common/pull/56

Full Changelog: https://github.com/mistralai/mistral-common/compare/v1.4.2...v1.4.3