Add LLM connector for Cohere

[rgriff23]

Justification

Cohere focuses on providing low-latency LLMs. They have an async endpoint we can use and free API keys.

Details

The new class will live in src/artkit/model/llm/cohere/

Be sure to update the docs:

• README: https://github.com/BCG-X-Official/artkit?tab=readme-ov-file#supported-model-providers
• Contributing: https://github.com/BCG-X-Official/artkit/blob/1.0.x/sphinx/source/user_guide/introduction_to_artkit/connecting_to_genai_models.ipynb
• Connecting to Gen AI Models: https://github.com/BCG-X-Official/artkit/blob/1.0.x/sphinx/source/user_guide/introduction_to_artkit/connecting_to_genai_models.ipynb
• .env_example: https://github.com/BCG-X-Official/artkit/blob/1.0.x/.env_example

Please see the tutorial: Creating New Model Classes