Add feature flipping between two different API

[MayuriXx]

We use if the random user API is down :

URL for Random User API

randomuser.api.base-url=https://dummyjson.com/ -> rename this, for example : dummy.api.base-url

So create a second one url for example : randomuser.api.base-url with the url of random-user : https://randomuser.me/

Update the RandomUserRetrofit
Update the documentation
Update fetchRandomUsers with first call at random user, if the response is 500 so call the second api

/!\ TWO models different, one for dummy and one for randomUser