Update ERC20 Token with Decimals & Permit support

[seaona]

Context: the old ERC20 token contract didn't allow for changing the decimals, when passed on the contract deployment. This has been showcasted here even if it looks like we pass a value, it always defaults to 4. This problematic, as we need the ability to set the decimals in order to test different cases (i.e. tokens with 0 decimals).

Furthermore, having an ERC20 with Permit support, will allow us to test the Permit signature flow end-to-end. The previous Permit signature work has already been done here.

Lastly, since we didn't have the source contract, we could not actually see any of these missing properties, until it has been manually checked. So, having the contract source is convenient.

What: this PR updates the ERC20 token contract with 3 improvements:

• Allowing to input decimals on the constructor, so 4decimals are not hardcoded anymore
• Adding Permit support. This will allow us to test end-to-end the Permit Signature, by grabbing the r, s and v values. Introduced already here
• Adding the contract source on the contracts folder

How: the ERC20 with Permit support has been created using the Open Zeppelin wizard, to make sure we are following the standards.
The decimal support on the constructor has been modified on the ERC20 contract base from open-zeppelin.

[PeterYinusa]

LGTM!