Add support for dynamic and nested NFTs

[aniket2405]

1. The modified contract introduces a new NftType enum to differentiate between static, dynamic, and nested NFTs. The NftData struct contains the new fields nftType, parentId, and childIds to support dynamic and nested NFTs.
2. The createStaticNft function creates a new static NFT with the specified recipient and token ID.
3. The createDynamicNft function creates a new dynamic NFT with a new token ID and the specified recipient.
4. The createNestedNft function creates a new nested NFT with a new token ID, the specified recipient, and the parent NFT ID. It also adds the child NFT ID to the childIds array of the parent NFT.
5. The getChildNfts function returns an array of child NFT IDs for a given parent NFT ID. (used to retrieve child NFTs for the nested NFTs)

[aniket2405]

@mbcse, modified the contract to support dynamic and nested NFTs.