[/scoring] add ability to define aggregation functions for scoring functions & refactors (#597)

# What does this PR do?

- Add ability to define aggregation functions for scoring functions via
`ScoringFnParams`
- Supported by `basic` / `regex_parser` / `llm_as_judge` scoring
functions


## Test Plan

```
pytest -v -s -m basic_scoring_together_inference scoring/test_scoring.py
```
<img width="855" alt="image"
src="https://github.com/user-attachments/assets/12db8e6e-2ad4-462e-b9b9-70ba6c050a6c">


```
pytest -v -s -m llm_as_judge_scoring_together_inference scoring/test_scoring.py
```
<img width="858" alt="image"
src="https://github.com/user-attachments/assets/bf806676-6f5e-456d-be9f-f81a26d1df19">



**Example Response** (`basic`)
<img width="863" alt="image"
src="https://github.com/user-attachments/assets/0e57a49c-8386-45cc-8fa9-3e61aaa9a3be">

**Example Response** (`llm-as-judge`)
<img width="854" alt="image"
src="https://github.com/user-attachments/assets/38065bc2-b724-47ed-9535-79b6099c4362">


## Sources

Please link relevant resources if necessary.


## 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/inline/scoring/braintrust/braintrust.py

@@ -147,7 +147,7 @@ class BraintrustScoringImpl(
                 await self.score_row(input_row, scoring_fn_id)
                 for input_row in input_rows
             ]
-
+            aggregation_functions = [AggregationFunctionType.average]
             agg_results = aggregate_average(score_results)
             res[scoring_fn_id] = ScoringResult(
                 score_rows=score_results,