Add support for nested NFTs

[aniket2405]

These are NFTs that can contain other NFTs as sub-assets. For example, a game item NFT may contain a separate NFT that represents a character's skin. To add support for nested NFTs, we must include logic for creating, managing, and accessing sub-assets within NFTs. We also need to allow for the creation of NFTs that contain other NFTs as properties or attributes. This would require changes to the data structures used to represent NFTs and the addition of functionality to manage the creation, ownership, and transfer of nested NFTs.