Feature/nft contract implementation

[ayomideadeniran]

Closes #20

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🎯 Goal

Implement a standards-compliant NFT smart contract using the Soroban SDK for the NFTopia ecosystem.

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🚀 Changes

• Project Setup: Configured Cargo.toml and workspace for nft_contract.
• Core Structures: Added TokenData, TokenAttribute, CollectionConfig, and Role.
• Storage Layout: Organized instance and persistent storage using DataKey.
• RBAC: Implemented Admin, Minter, Burner, and Metadata Updater roles with secure access checks.
• Token Logic: Added mint, burn, transfer, and operator approvals.
• Metadata: Collection config updates, token URI handling, and metadata freezing.
• Royalties: On-chain royalty calculation (EIP-2981 equivalent).
• Batch Ops: Implemented batch_mint and batch_transfer.
• Testing: Added unit and integration tests covering token workflows and royalty validation.

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✅ Verification

• All tests pass (cargo test)
• Code formatted (cargo fmt)
• Implemented across 12 structured commits

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[ayomideadeniran]

Please review this Pr