Add customizable fee receiver to ERC20FlashMint (#3327)

Co-authored-by: Mazen Khalil <mazen@immunityledger.org>
Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/token/ERC20/extensions/ERC20FlashMint.sol

@@ -43,6 +43,16 @@ abstract contract ERC20FlashMint is ERC20, IERC3156FlashLender {
         return 0;
     }
 
+    /**
+     * @dev Returns the receiver address of the flash fee. By default this
+     * implementation returns the address(0) which means the fee amount will be burnt.
+     * This function can be overloaded to change the fee receiver.
+     * @return The address for which the flash fee will be sent to.
+     */
+    function _flashFeeReceiver() internal view virtual returns (address) {
+        return address(0);
+    }
+
     /**
      * @dev Performs a flash loan. New tokens are minted and sent to the
      * `receiver`, who is required to implement the {IERC3156FlashBorrower}
@@ -73,8 +83,14 @@ abstract contract ERC20FlashMint is ERC20, IERC3156FlashLender {
             receiver.onFlashLoan(msg.sender, token, amount, fee, data) == _RETURN_VALUE,
             "ERC20FlashMint: invalid return value"
         );
+        address flashFeeReceiver = _flashFeeReceiver();
         _spendAllowance(address(receiver), address(this), amount + fee);
-        _burn(address(receiver), amount + fee);
+        if (fee == 0 || flashFeeReceiver == address(0)) {
+            _burn(address(receiver), amount + fee);
+        } else {
+            _burn(address(receiver), amount);
+            _transfer(address(receiver), flashFeeReceiver, fee);
+        }
         return true;
     }
 }