Enable custom token counting

[luisquintanilla]

Currently, Tokenizer returns counts strictly based on tokens.

However, there are scenarios where library authors may want / need to implement their own custom token counting function.

One such scenario is providing token counts for image inputs. In such cases, AI service providers provide an arbitrary way of calculating cost based on a fixed token count.

Provider	Cost calculations	Does tokenization	Link
OpenAI	Fixed	No	https://platform.openai.com/docs/guides/vision/calculating-costs
Claude	Fixed	No	https://docs.anthropic.com/en/docs/build-with-claude/vision#calculate-image-costs
Gemini	Fixed	No	https://ai.google.dev/gemini-api/docs/tokens?lang=python#multimodal-tokens
Cohere	N/A	N/A	https://docs.cohere.com/docs/tokens
Mistral	N/A	N/A	https://docs.mistral.ai/guides/tokenization/#tokens-count