add buyCollateral script and add setTargetReserves function for lending contract

contracts/interfaces/ILending.sol

@@ -14,6 +14,7 @@ interface ILending {
     event AbsorbCollateral(address indexed absorber, address indexed borrower, address indexed asset, uint256 collateralAbsorbed, uint256 usdValue);
     event BuyCollateral(address indexed buyer, address indexed asset, uint256 baseAmount, uint256 collateralAmount);
     event WithdrawReserves(address indexed to, uint256 amount);
+    event TargetReservesUpdated(uint104 targetReserves);
     
     error Unauthorized();
     error InsufficientBalance();

contracts/ytLending/Lending.sol

@@ -95,6 +95,11 @@ contract Lending is
         _unpause();
     }
 
+    function setTargetReserves(uint104 newTargetReserves) external onlyOwner {
+        targetReserves = newTargetReserves;
+        emit TargetReservesUpdated(targetReserves);
+    }
+
     /**
      * @notice 计算累计利息后的索引（不修改状态）
      * @param timeElapsed 经过的时间