fix: add Ownable.sol smart contract

.gitignore

@@ -12,7 +12,6 @@
 *.out
 
 # Dependency directories (remove the comment below to include it)
-vendor/
 .vscode/
 
 # Environment files

eth-contracts/contracts/vendor/Ownable.sol

@@ -0,0 +1,66 @@
+//SPDX-License-Identifier: MIT
+pragma solidity >= 0.5.0 < 0.7.0;
+
+/**
+ * @dev Contract module which provides a basic access control mechanism, where
+ * there is an account (an owner) that can be granted exclusive access to
+ * specific functions.
+ *
+ * This module is used through inheritance. It will make available the modifier
+ * `onlyOwner`, which can be aplied to your functions to restrict their use to
+ * the owner.
+ *
+ * This contract has been modified to remove the revokeOwnership function
+ */
+contract Ownable {
+  address private _owner;
+
+  event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);
+
+  /**
+   * @dev Initializes the contract setting the deployer as the initial owner.
+   */
+  constructor () internal {
+    _owner = msg.sender;
+    emit OwnershipTransferred(address(0), _owner);
+  }
+
+  /**
+   * @dev Returns the address of the current owner.
+   */
+  function owner() public view returns (address) {
+    return _owner;
+  }
+
+  /**
+   * @dev Throws if called by any account other than the owner.
+   */
+  modifier onlyOwner() {
+    require(isOwner(), "Ownable: caller is not the owner");
+    _;
+  }
+
+  /**
+   * @dev Returns true if the caller is the current owner.
+   */
+  function isOwner() public view returns (bool) {
+    return msg.sender == _owner;
+  }
+
+  /**
+   * @dev Transfers ownership of the contract to a new account (`newOwner`).
+   * Can only be called by the current owner.
+   */
+  function transferOwnership(address newOwner) public onlyOwner {
+    _transferOwnership(newOwner);
+  }
+
+  /**
+   * @dev Transfers ownership of the contract to a new account (`newOwner`).
+   */
+  function _transferOwnership(address newOwner) internal {
+    require(newOwner != address(0), "Ownable: new owner is the zero address");
+    emit OwnershipTransferred(_owner, newOwner);
+    _owner = newOwner;
+  }
+}