Implement Soroban Provider Health Aggregator

# 🧠 Concept
Aggregate health metrics across bridge providers.

## ⚠️ Problem
Provider health data is fragmented.

## 📁 Implementation Scope
- `src/monitoring/providers/stellar/`

## ✅ Requirements
- Collect health metrics
- Normalize provider status
- Generate health summaries

## 🎯 Acceptance Criteria
- [ ] Health aggregator implemented
- [ ] Provider summaries available