Support the Anthropic Token Counting API

[areibman]

Recent release: https://docs.anthropic.com/en/docs/build-with-claude/token-counting

We currently count tokens according to the GPT tiktoken tokenizer. We should use the API instead

[dot-agi]

Currently, we can update the count_message_tokens function to use this API.

count_string_tokens will use the old estimation.

[dot-agi]

Currently, we can update the count_message_tokens function to use this API.

count_string_tokens will use the old estimation.

They removed client.count_tokens in version 0.39.0 (Source) so we cannot use count_string_tokens anymore.