feat(soroban): Implement NFT certificate contract

[zeemscript]

Description

Create a Soroban smart contract for minting NFT certificates as proof of course completion. These on-chain certificates serve as verifiable credentials that students can showcase as proof of their educational achievements.

Acceptance Criteria

• Mint unique NFT certificates upon course completion
• Include metadata (course name, completion date, student ID)
• Support certificate verification by contract ID and token ID
• Integration with course completion system

Files to modify/create

• contracts/nft-certificate/src/lib.rs
• contracts/nft-certificate/Cargo.toml
• src/modules/stellar/certificate.service.ts
• Metadata schema documentation