Improvement getNextExtranonce2 and create getRandomExtranonce2

[AchcarLucas]

I was trying to modify the getNextExtranonce2 function to have a random nonce and like the code I made It was a little better than the code that was before (I believe), I decided to create a merge while maintaining the function original with the modification and I also created a getRandomExtranonce2 function that can be used in the future or even make the change through some configuration

[BitMaker-hub]

Great! I like it too, thanks for your contribution