Added piggyBank contract example

[Nonnyjoe]

As requested, I've added the complete piggyBank contract along with it's complete tests (normal and fork tests) to the Example folder

[Nonnyjoe]

@gianalarcon

[gianalarcon]

Awesome. Great job.
I will add some requirements:

• Starknet-foundry version should 0.13.1 (or at least 0.13.0)
• starknet = "2.4.1" (or at least "2.4.0")
  Optional
  It would be great if you can use a sepolia rpc-url instead of goerli. You can check some providers here

[Nonnyjoe]

Thanks for the review @gianalarcon, as requested all updates has been effected.