Feature Request: Support Anthropic messages API

[onlurking]

The new Claude 3 model family by Anthropic (claude-3-sonnet, claude-3-opus, and claude-3-haiku in the future) is only available in the /messages endpoint.

Usage example:

curl https://api.anthropic.com/v1/messages \
     --header "x-api-key: $ANTHROPIC_API_KEY" \
     --header "anthropic-version: 2023-06-01" \
     --header "content-type: application/json" \
     --data \
'{
    "model": "claude-3-opus-20240229",
    "max_tokens": 1024,
    "messages": [
        {"role": "user", "content": "Hello, world"}
    ]
}'