[FT] Add System Prompt field in LightevalTaskConfig that can be used by model clients

## Issue encountered
My models currently don't follow the template I give. I want to give a system prompt that nudges the models to provide output the way I want it.

## Solution/Feature
We could add a new field in LightevalTaskConfig that can be consumed by model clients like LiteLLMClient to send to the API.

## Possible alternatives
Alternatively, we could use the generation_grammar, but I think this may be a bit overkill and may be more difficult to implement for different API providers.