fix: Return final withdraw amount in L2Pool withdraw fn (#831)

Author: miguelmtzinf

contracts/interfaces/IL2Pool.sol

@@ -39,8 +39,9 @@ interface IL2Pool {
    * @dev the shortenedAmount is cast to 256 bits at decode time, if type(uint128).max the value will be expanded to
    * type(uint256).max
    * @dev assetId is the index of the asset in the reservesList.
+   * @return The final amount withdrawn
    */
-  function withdraw(bytes32 args) external;
+  function withdraw(bytes32 args) external returns (uint256);
 
   /**
    * @notice Calldata efficient wrapper of the borrow function, borrowing on behalf of the caller

contracts/protocol/pool/L2Pool.sol

@@ -40,10 +40,10 @@ contract L2Pool is Pool, IL2Pool {
   }
 
   /// @inheritdoc IL2Pool
-  function withdraw(bytes32 args) external override {
+  function withdraw(bytes32 args) external override returns (uint256) {
     (address asset, uint256 amount) = CalldataLogic.decodeWithdrawParams(_reservesList, args);
 
-    withdraw(asset, amount, msg.sender);
+    return withdraw(asset, amount, msg.sender);
   }
 
   /// @inheritdoc IL2Pool