Add extended task for LiveCodeBench codegeneration (#548)
```shell
lighteval vllm \
"pretrained=deepseek-ai/DeepSeek-R1-Distill-Qwen-1.5B,dtype=float16,data_parallel_size=4,max_model_length=32768,gpu_memory_utilisation=0.8,generation_parameters={temperature: 0.7}" \
"extended|lcb:codegeneration|0|0" \
--use-chat-template
```
