Refactor transferCrossChain to support been extended by user in zetachain/UniversalNFTCore.sol

[andresaiello]

Right now transferCrossChain in UniversalNFTCore it's like this:

 function transferCrossChain(
        uint256 tokenId,
        address receiver,
        address destination
    ) public payable {
// code

then if developer use it out of the box it's not using the gas in an efficient way because the calls will be externals. But to refactor to external should consider another refactor also like mentioned in issue #45

I recommend to refactor like this:

 function transferCrossChain(
        uint256 tokenId,
        address receiver,
        address destination
    ) external payable {
_transferCrossChain(     tokenId,
        receiver,
        destination);
}

 function _transferCrossChain(
        uint256 tokenId,
        address receiver,
        address destination
    ) internal virtual{
// code
}

then if a developer wants to extend it it's just an internal call