Implement Soroban Transfer Cost Forecasting

# 🧠 Concept
Forecast future transfer costs using historical data.

## ⚠️ Problem
Users lack insight into changing bridge costs.

## 📁 Implementation Scope
- `src/forecasting/costs/stellar/`

## ✅ Requirements
- Analyze fee trends
- Predict future costs
- Generate forecasts

## 🎯 Acceptance Criteria
- [ ] Cost forecasting implemented
- [ ] Forecasts generated successfully