Support Anthropic Messages API

What does this merge request do and why?

Support Anthropic Messages API. Anthropic Completions API is deprecated and Claude 3 models only work with Messages API moving forward.

Please note that this MR does not include the wiring in the API layer.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.

Closes #419 (closed)