feat: implement SablierFees contract

src/SablierFees.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity >=0.8.22;
+
+import { ISablierFees } from "./interfaces/ISablierFees.sol";
+import { Errors } from "./libraries/Errors.sol";
+
+import { Adminable } from "./Adminable.sol";
+
+/// @title SablierFees
+/// @notice See the documentation in {ISablierFees}.
+abstract contract SablierFees is Adminable, ISablierFees {
+    /// @inheritdoc ISablierFees
+    function collectFees() external override {
+        uint256 feeAmount = address(this).balance;
+
+        // Effect: transfer the fees to the admin.
+        (bool success,) = admin.call{ value: feeAmount }("");
+
+        // Revert if the call failed.
+        if (!success) {
+            revert Errors.SablierFees_FeeTransferFail(admin, feeAmount);
+        }
+
+        // Log the fee withdrawal.
+        emit ISablierFees.CollectFees({ admin: admin, feeAmount: feeAmount });
+    }
+}

src/interfaces/ISablierFees.sol

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+// SPDX-License-Identifier: GPL-3.0-or-later
+pragma solidity >=0.8.22;
+
+/// @notice This contract handles the logic for managing Sablier fees.
+interface ISablierFees {
+    /// @notice Emitted when the accrued fees are collected.
+    /// @param admin The address of the current contract admin, which has received the fees.
+    /// @param feeAmount The amount of collected fees.
+    event CollectFees(address indexed admin, uint256 indexed feeAmount);
+
+    /// @notice Collects the accrued fees by transferring them to the contract admin.
+    ///
+    /// @dev Emits a {CollectFees} event.
+    ///
+    /// Notes:
+    /// - If the admin is a contract, it must be able to receive ETH.
+    function collectFees() external;
+}