UniversalNFTCore doesn't follow ERC7201 namespaced storage pattern

[lukema95]

The UniversalNFTCore contract in @zetachain/standard-contracts doesn't follow the ERC7201 namespaced storage pattern, which can lead to storage slot conflicts and upgrade compatibility issues in proxy-based upgradeable contracts.

Refactor the storage layout to use ERC7201 namespaced storage pattern:

abstract contract UniversalNFTCore is
    ERC721Upgradeable,
    ERC721URIStorageUpgradeable,
    OwnableUpgradeable,
    UniversalNFTEvents
{
    /// @custom:storage-location erc7201:zetachain.storage.UniversalNFTCore
    struct UniversalNFTCoreStorage {
        /// @dev Address of the EVM gateway contract
        GatewayEVM gateway;
        /// @dev The address of the Universal NFT contract on ZetaChain
        address universal;
        /// @dev The amount of gas used when making cross-chain transfers
        uint256 gasLimitAmount;
    }

    // keccak256(abi.encode(uint256(keccak256("zetachain.storage.UniversalNFTCore")) - 1)) & ~bytes32(uint256(0xff))
    bytes32 private constant UniversalNFTCoreStorageLocation = 0x...;

    function _getUniversalNFTCoreStorage() private pure returns (UniversalNFTCoreStorage storage $) {
        assembly {
            $.slot := UniversalNFTCoreStorageLocation
        }
    }

    // Update all getter functions to use the storage struct
    function gateway() public view returns (GatewayEVM) {
        UniversalNFTCoreStorage storage $ = _getUniversalNFTCoreStorage();
        return $.gateway;
    }

    function universal() public view returns (address) {
        UniversalNFTCoreStorage storage $ = _getUniversalNFTCoreStorage();
        return $.universal;
    }

    function gasLimitAmount() public view returns (uint256) {
        UniversalNFTCoreStorage storage $ = _getUniversalNFTCoreStorage();
        return $.gasLimitAmount;
    }

    // Update all setter functions accordingly...
}