[Agentic Eval] add ability to run agents generation (#469)

# What does this PR do?

- add ability to run agents generation for full eval (generate +
scoring)
- pre-register SimpleQA  benchmark llm-as-judge scoring function in code


## Test Plan


![image](https://github.com/user-attachments/assets/b4b6f086-1be4-4c2a-8ab0-6839f0067c0a)


![image](https://github.com/user-attachments/assets/05bb7a09-2d7a-4031-8eb6-e1ca670ee439)


#### Simple QA w/ Search

![image](https://github.com/user-attachments/assets/0a51e3f3-9fc7-479b-8295-89aed63496e0)

- eval_task_config_simpleqa_search.json
```json
{
    "type": "benchmark",
    "eval_candidate": {
        "type": "agent",
        "config": {
            "model": "Llama3.1-405B-Instruct",
            "instructions": "Please use the search tool to answer the question.",
            "sampling_params": {
                "strategy": "greedy",
                "temperature": 1.0,
                "top_p": 0.9
            },
            "tools": [
                {
                    "type": "brave_search",
                    "engine": "brave",
                    "api_key": "API_KEY"
                }
            ],
            "tool_choice": "auto",
            "tool_prompt_format": "json",
            "input_shields": [],
            "output_shields": [],
            "enable_session_persistence": false
        }
    }
}
```

#### SimpleQA w/o Search

![image](https://github.com/user-attachments/assets/6301feef-2abb-4bee-b50c-97da1c90482b)


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/registry/eval.py

@@ -22,6 +22,7 @@ def available_providers() -> List[ProviderSpec]:
                 Api.datasets,
                 Api.scoring,
                 Api.inference,
+                Api.agents,
             ],
         ),
     ]