Verify smart contract request with multi-signature approach

First, create a mintable token

// SPDX-License-Identifier: MIT
// @nhancv
pragma solidity 0.8.4;

interface IMintable {
  function mint(address account, uint256 amount) external;
}

ERC20Mintable

// SPDX-License-Identifier: MIT
// @nhancv
pragma solidity 0.8.4;

import "@openzeppelin/contracts-upgradeable/access/AccessControlUpgradeable.sol";
import "./ERC20Token.sol";
import "./interfaces/IMintable.sol";

contract ERC20Mintable is ERC20Token, IMintable, AccessControlUpgradeable {
  bytes32 public constant MINTER_ROLE = keccak256("MINTER_ROLE");

  /**
   * @dev Upgradable initializer
   */
  function __ERC20Mintable_init(
    string memory name_,
    string memory symbol_,
    uint8 decimals_,
    uint256 initialSupply_,
    address minter_
  ) public initializer {
    __ERC20Token_init(name_, symbol_, decimals_, initialSupply_);
    _setupRole(DEFAULT_ADMIN_ROLE, _msgSender());
    _setupRole(MINTER_ROLE, minter_);
  }

  /**
   * @dev Mint tokens.
   */
  function mint(address _to, uint256 _amount) public override onlyRole(MINTER_ROLE) {
    _mint(_to, _amount);
  }
}

Create EIP712 multi-signature

MultiSigEIP712

// SPDX-License-Identifier: MIT
// @nhancv
pragma solidity ^0.8.4;

import "@openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol";
import "@openzeppelin/contracts-upgradeable/access/OwnableUpgradeable.sol";
import "@openzeppelin/contracts-upgradeable/access/AccessControlUpgradeable.sol";
import "@openzeppelin/contracts-upgradeable/security/ReentrancyGuardUpgradeable.sol";
import "@openzeppelin/contracts-upgradeable/utils/cryptography/draft-EIP712Upgradeable.sol";
import "@openzeppelin/contracts-upgradeable/utils/cryptography/ECDSAUpgradeable.sol";
import "./interfaces/IMintable.sol";

contract MultiSigEIP712 is EIP712Upgradeable, OwnableUpgradeable, AccessControlUpgradeable, ReentrancyGuardUpgradeable {
  // A valid validator must has VALIDATOR_ROLE
  bytes32 public constant VALIDATOR_ROLE = keccak256("VALIDATOR_ROLE");
  // A valid request requires at least validatorsRequired
  uint256 public validatorsRequired;

  // Request entries
  struct ProcessedEntry {
    uint256 processId;
    address user;
    address token;
    uint256…