Add a way to easily enable/disable thinking mode for reasoning models

## Issue encountered
When evaluating reasoning capable models (e.g., Qwen3, etc) with lighteval, there's currently no way to toggle enable_thinking directly via CLI or config.

Is there a way to work around this or does it require a PR ?

## Solution/Feature
Maybe add a flag or config option to enable/disable thinking mode (i.e., pass enable_thinking=False to the tokenizer) for models that support it.