feat: Implement a comprehensive, standards-compliant NFT contract

[raizo07]

Closes #342

I've Implemented a full Soroban NFT contract module set with roles, mint/burn/transfer/approval, metadata + royalties, batch ops, pause/whitelist/reveal controls, events, interface detection, and basic tests under nftopia-stellar/contracts/nft_contract/src.

The contract now has explicit storage/layout and helpers to support provenance tracking and safe transfers with receiver validation.

Key changes and locations:

• Core contract and public API in nftopia-stellar/contracts/nft_contract/src/lib.rs.
• Storage, roles, metadata, royalty, transfers, events, and interface helpers split into src/storage.rs, src/access_control.rs, * src/metadata.rs, src/royalty.rs, src/transfer.rs, src/events.rs, src/interface.rs, src/utils.rs.
• Data structures in src/token.rs and error types in src/error.rs.
• Basic unit tests in src/test.rs.
• Updated Soroban dependencies and lib config in nftopia-stellar/contracts/nft_contract/Cargo.toml.

Notes:
safe_transfer_from calls on_nft_received on contract recipients and requires true to proceed.
token_uri falls back to base_uri when unrevealed or when token URI is empty.
Royalties are computed via get_royalty_info (EIP-2981-style); actual payment remains a marketplace responsibility.

[Oluwaseyi89]

@raizo07 please pull our main branch and push again to get updated.

[raizo07]

@Oluwaseyi89 Done

[Oluwaseyi89]

@raizo07 your PR failed checks.

[Oluwaseyi89]

@raizo07 checks failed again. Please, review your code and ensure it builds successfully and passes all tests.