ERC721Enumerable - Add back `_tokensOfOwner()`

tokensOfOwner() was first suggest on #1512 and implemented on #1522 and was removed in a bulk edit that makes me wonder if it was removed accidentally. https://github.com/OpenZeppelin/openzeppelin-contracts/commit/bd0778461da82364aebd2d763bb338795c5f9fa7

This function helps when coding web3 apps. Instead of calling the RPC multiple times to get all NFTs owned by the user, it can be done at once by calling tokensOfOwner()

[imsys]

So, I just tested this, and it works!

I know it is an unbounded loop, but it doesn’t seem to be a problem for a RPC.

pragma solidity ^0.8.0;

// SPDX-License-Identifier: MIT

import "@openzeppelin/contracts/token/ERC721/extensions/IERC721Enumerable.sol";

contract ListNFT {

    function listUserNFTs(address contractAddress, address owner) external view returns (uint[] memory) {

        uint balance = IERC721Enumerable(contractAddress).balanceOf(owner);

        uint[] memory tokens = new uint[](balance);

        for (uint i=0; i<balance; i++) {
            tokens[i] = (IERC721Enumerable(contractAddress).tokenOfOwnerByIndex(owner, i));
        }

        return tokens;
    }
}

[imsys]

For me this can be closed then, unless someone else has anything else to add.