feat: ZetaChain.omniChainSwap() function

[fadeev]

Makes it possible to fit an omnichain swap contract in a tweet:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;

import "@zetachain/toolkit/contracts/ZetaChain.sol";

contract Swap is ZetaChain {
    constructor() ZetaChain() {}

    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external override onlySystem {
        ZetaChain.omniChainSwap(context, zrc20, amount, message);
    }
}

Depends on #134