Implement Stellar Fee Estimation Engine

# 🧠 Concept  
Estimate Stellar smart contract execution fees.

## ⚠️ Problem  
Developers lack accurate execution cost predictions.

## 📁 Implementation Scope  
- `packages/gas-estimator/stellar/`

## ✅ Requirements  
- Estimate execution/storage fees  
- Support simulation-based calculations  

## 🎯 Acceptance Criteria  
- [ ] Fee estimator implemented  
- [ ] Estimates validated  