Add refresh token model

src/libs/models/refreshToken.js

+const mongoose = require('mongoose');
+const Schema = mongoose.Schema;
+const libs = `${process.cwd()}/libs`;
+const User = require(`${libs}/models/user`);
+
+let RefreshToken = new Schema({
+    userId: {
+        type: Schema.Types.ObjectId,
+        required: true,
+        ref: 'User'
+    },
+    token: {
+        type: String,
+        unique: true,
+        required: true
+    },
+    createdAt: {
+        type: Date,
+        default: Date.now
+    }
+});
+
+module.exports = mongoose.model('RefreshToken', RefreshToken);
+