How to Build a Lottery Contract using Oracle Randomness

I have gone through the posts in Celo tutorial and found that there was no tutorial for working with randomness from an oracle. So here in this tutorial viewers will find how to use a Random Number Generator and applying it practically by building a small lottery contract. Witnet will be used for this tutorial. The tutorial first will also go through the problem with on chain randomness , what is an oracle and random number generation, and the walkthrough for the contract. I have added the sample into my previous works.