feat(pool-monitor): add initial liquidity monitor

packages/contracts/src/dollar/monitors/PoolLiquidityMonitor.sol

@@ -0,0 +1,38 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.19;
+
+import "../facets/UbiquityPoolFacet.sol";
+import "@openzeppelin/contracts/access/Ownable.sol";
+
+contract PoolLiquidityMonitor is Ownable {
+    UbiquityPoolFacet public ubiquityPoolFacet;
+    address public defenderRelayer;
+
+    event LiquidityChecked(uint256 currentLiquidity);
+
+    constructor(address _ubiquityPoolFacetAddress, address _defenderRelayer) {
+        ubiquityPoolFacet = UbiquityPoolFacet(_ubiquityPoolFacetAddress);
+        defenderRelayer = _defenderRelayer;
+    }
+
+    modifier onlyAuthorized() {
+        require(
+            msg.sender == defenderRelayer,
+            "Not authorized: Only Defender Relayer allowed"
+        );
+        _;
+    }
+
+    function setDefenderRelayer(
+        address _newDefenderRelayer
+    ) external onlyOwner {
+        defenderRelayer = _newDefenderRelayer;
+    }
+
+    function checkLiquidity() external onlyAuthorized {
+        uint256 currentCollateralLiquidity = ubiquityPoolFacet
+            .collateralUsdBalance();
+
+        emit LiquidityChecked(currentCollateralLiquidity);
+    }
+}