Implement Soroban Contract Benchmark Runner

# 🧠 Concept  
Benchmark Soroban contract execution performance.

## ⚠️ Problem  
Developers lack measurable optimization insights.

## 📁 Implementation Scope  
- `tests/performance/stellar/`

## ✅ Requirements  
- Run execution benchmarks  
- Generate performance reports  

## 🎯 Acceptance Criteria  
- [ ] Benchmark runner implemented  
- [ ] Reports generated  