feat(soroban): Add crowdfunding campaign contract

[zeemscript]

Description

Develop a Soroban smart contract for goal-based crowdfunding campaigns with automatic refunds if the funding goal is not met. This enables community-funded educational initiatives with built-in trust and transparency.

Acceptance Criteria

• Set funding goal and deadline for campaigns
• Accept contributions from multiple backers
• Automatic refund mechanism if goal not reached by deadline
• Release funds to campaign creator upon successful completion

Files to modify/create

• contracts/crowdfunding/src/lib.rs
• contracts/crowdfunding/Cargo.toml
• src/modules/stellar/crowdfunding.service.ts
• src/modules/stellar/entities/campaign.entity.ts