feat: add interest model

solidity/contracts/money-market/interest-models/MMFlatSlopeModel3.sol

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: BUSL
+pragma solidity 0.8.19;
+
+contract MMFlatSlopeModel3 {
+  /// @dev Return the interest rate per second, using 1e18 as denom.
+  function getInterestRate(
+    uint256, /*debt*/
+    uint256 /*floating*/
+  ) external pure returns (uint256) {
+    // 8% flat rate = 8e16 / 365 days
+    return 2536783358;
+  }
+}

solidity/tests/money-market/interest-models/MMFlatSlope3.t.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+pragma solidity 0.8.19;
+
+import { BaseTest, console } from "../../base/BaseTest.sol";
+
+import { MMFlatSlopeModel3 } from "../../../contracts/money-market/interest-models/MMFlatSlopeModel3.sol";
+
+// solhint-disable func-name-mixedcase
+// solhint-disable contract-name-camelcase
+contract MMFlatSlopeModel3_Test is BaseTest {
+  MMFlatSlopeModel3 private _flatSlopeModel3;
+
+  function setUp() external {
+    _flatSlopeModel3 = new MMFlatSlopeModel3();
+  }
+
+  function _findInterestPerYear(uint256 _interestPerSec) internal pure returns (uint256) {
+    return _interestPerSec * 365 days;
+  }
+
+  function testFuzz_getInterestRate(uint256 debt, uint256 floating) external {
+    // when utilization is whatever, interest will always be 7.99% ~ 8.00%
+    assertEq(_findInterestPerYear(_flatSlopeModel3.getInterestRate(debt, floating)), 0.079999999977888000 ether);
+  }
+}