feat: Add save/load functionality and improved repr for LLM-based metrics (#2320)

## Summary

This PR adds persistence capabilities and better string representations
for LLM-based metrics, making them easier to save, share, and debug.

## Changes

### 1. Save/Load Functionality
- Added `save()` and `load()` methods to `SimpleLLMMetric` and its
subclasses (`DiscreteMetric`, `NumericMetric`, `RankingMetric`)
- Supports JSON format with optional gzip compression
- Handles all prompt types including `Prompt` and `DynamicFewShotPrompt`
- Smart defaults: `metric.save()` saves to `./metric_name.json`

### 2. Improved `__repr__` Methods
- Clean, informative string representations for both LLM-based and
decorator-based metrics
- Removed implementation details (memory addresses, `<locals>`, internal
attributes)
- Smart prompt truncation (80 chars max)
- Function signature display for decorator-based metrics

**Before:**
```python
create_metric_decorator.<locals>.decorator_factory.<locals>.decorator.<locals>.CustomMetric(name='summary_accuracy', _func=<function summary_accuracy at 0x151ffdf80>, ...)
```

**After:**
```python
# LLM-based metrics
DiscreteMetric(name='response_quality', allowed_values=['correct', 'incorrect'], prompt='Evaluate if the response...')

# Decorator-based metrics  
summary_accuracy(user_input, response) -> DiscreteMetric[['pass', 'fail']]
```

### 3. Response Model Handling
- Added `create_auto_response_model()` factory to mark auto-generated
models
- Only warns about custom response models during save, not standard ones

## Usage Examples

```python
# Save metric with default path
metric.save()  # → ./response_quality.json

# Save with custom path
metric.save("custom.json")
metric.save("/path/to/metrics/")  # → /path/to/metrics/response_quality.json
metric.save("compressed.json.gz")  # Compressed

# Load metric
loaded_metric = DiscreteMetric.load("response_quality.json")

# For DynamicFewShotPrompt metrics
loaded_metric = DiscreteMetric.load("metric.json", embedding_model=embeddings)
```

## Testing
- Comprehensive test suite with 8 tests covering all save/load scenarios
- Tests for default paths, directory handling, compression
- Tests for all prompt types and metric subclasses

## Dependencies
**Note:** This PR builds on #2316 (Fix metric inheritance patterns) and
requires it to be merged first. The changes here depend on the
cleaned-up metric inheritance structure from that PR.

## Checklist
- [x] Tests added
- [x] Documentation in docstrings
- [x] Backwards compatible (new functionality only)
- [x] Follows TDD practices

Author: jjmachan