Implement ERC721 metadata

[leoloco]

Context

The current NFT standard as developed in this repo only implements the interface defined in the EIP 721 https://eips.ethereum.org/EIPS/eip-721

Since the definition of this interface, the community has developed various extensions that extend the functionalities of NFT.
One of the most commonly used extension is the metadata extension: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/extensions/IERC721Metadata.sol

It allows one to associate every token with a URI that contains the metadata of the corresponding NFT.

User Flow

As an NFT creator (minter), I want to be able to assign metadata to each NFT I create, so that each token can have distinct characteristics, images, or attributes defined in a JSON file hosted at a URI. This metadata enhances the uniqueness and collectibility of each NFT.

• As a creator, when I mint an NFT, I should be able to specify a metadata URI that points to a JSON file conforming to the ERC721 Metadata JSON Schema.
• As a collector, when I query an NFT's details, I expect to retrieve its metadata URI and be able to view the associated metadata (such as image, description, attributes) by accessing this URI.

Technical Details

• Implement NFT-metadata-internals

  • Create a datastructure to store URI
  • Create a method to set URI _setURI(uint256 tokenId, string URI)
  • Create a method to get URI _tokenURI(uint256 tokenId) returns (string);

• Implement an NFT-metadata-example
  Create an example implementation of a NFT-metadata