Add Cohere LLM integration

[vblagoje]

Summary and motivation

Add support for CohereGenerator and CohereChatGenerator.

Detailed design

Cohere is one of the leading LLM providers and we should have Haystack Cohere LLM integration. See https://docs.cohere.com/reference/generate and https://docs.cohere.com/reference/chat for more details.

Checklist

If the request is accepted, ensure the following checklist is complete before closing this issue.

• The code was merged in the main branch
• Docs are published at https://docs.haystack.deepset.ai/ (Chat generator missing)
• There is a Github workflow running the tests for the integration nightly and at every PR
• A label named like integration:<your integration name> has been added to this repo
• The labeler.yml file has been updated
• The package has been released on PyPI
• Update Cohere Page with CohereChatGenerator information haystack-integrations#150
• The integration has been listed in the Inventory section of this repo README
• There is an example available to demonstrate the feature
• CohereChatGenerator: The feature was announced through social media #265