Implement 'Smart Order Routing' for Multi-Asset Funding

[gabito1451]

Context

Accepting any asset for any project.

Problem

If a project wants USDC but the user has EURC, the swap cost might be high.

Suggested Execution

• Integrate Stellar's 'PathPayment' to find the best route from UserAsset to ProjectAsset.
• Propose the route during the 'InvestmentIntent' phase.

Acceptance Criteria

• Lowest possible fees for cross-asset investments.
• High conversion for diverse holders.

References

• backend/src/stellar/pathfinder.service.ts