Enable custom token counting

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 |