create user authentication schema

backend/lib/schemas/authentication.js

+'use strict'
+
+const Joi = require('joi')
+
+const userAuth = Joi.object({
+    user_email: Joi.string(),
+    created_at: Joi.date(),
+    updated_at: Joi.date(),
+    saltedhash: Joi.string()
+}).label('user_auth')
+
+module.exports = {
+    userAuth
+}