feat: simplify the omnichain template

[fadeev]

Before:

// SPDX-License-Identifier: MIT
pragma solidity 0.8.7;
 
import "@zetachain/protocol-contracts/contracts/zevm/SystemContract.sol";
import "@zetachain/protocol-contracts/contracts/zevm/interfaces/zContract.sol";
import "@zetachain/toolkit/contracts/OnlySystem.sol";
 
contract MyContract is zContract, OnlySystem {
    SystemContract public systemContract;
 
    constructor(address systemContractAddress) {
        systemContract = SystemContract(systemContractAddress);
    }
 
    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external virtual override onlySystem(systemContract) {
        // TODO: implement the logic
    }
}

After:

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

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

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

    function onCrossChainCall(
        zContext calldata context,
        address zrc20,
        uint256 amount,
        bytes calldata message
    ) external override onlySystem {
        // TODO: implement the logic
    }
}