Add OpenZeppelin contracts

package-lock.json

@@ -857,6 +857,12 @@
         "@types/web3": "1.0.19"
       }
     },
+    "@openzeppelin/contracts": {
+      "version": "4.3.2",
+      "resolved": "https://registry.npmjs.org/@openzeppelin/contracts/-/contracts-4.3.2.tgz",
+      "integrity": "sha512-AybF1cesONZStg5kWf6ao9OlqTZuPqddvprc0ky7lrUVOjXeKpmQ2Y9FK+6ygxasb+4aic4O5pneFBfwVsRRRg==",
+      "dev": true
+    },
     "@resolver-engine/core": {
       "version": "0.3.3",
       "resolved": "https://registry.npmjs.org/@resolver-engine/core/-/core-0.3.3.tgz",

package.json

@@ -5,6 +5,7 @@
   "devDependencies": {
     "@nomiclabs/hardhat-ethers": "^2.0.2",
     "@nomiclabs/hardhat-waffle": "^2.0.1",
+    "@openzeppelin/contracts": "^4.3.2",
     "chai": "^4.3.4",
     "ethereum-waffle": "^3.4.0",
     "ethers": "^5.4.7",