feat: add xdai gateway contracts

Author: shalzz

contracts/src/gateway/xdai/EthereumGateway.sol

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../bridge/xdai/xDaiL1Bridge.sol";
+import "../../bridge/xdai/interfaces/IAMB.sol";
+
+import "../interfaces/IForeignGateway.sol";
+import "../BaseForeignGateway.sol";
+
+contract EthereumGateway is BaseForeignGateway, xDaiL1Bridge {
+    constructor(
+        uint256 _arbitrationCost,
+        IHomeGateway _homeGateway,
+        address _l2Target,
+        IAMB _amb
+    ) BaseForeignGateway(_arbitrationCost, _homeGateway) xDaiL1Bridge(_l2Target, _amb) {}
+}

contracts/src/gateway/xdai/xDaiGateway.sol

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+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../arbitration/IArbitrator.sol";
+import "../../bridge/xdai/xDaiL2Bridge.sol";
+import "../../bridge/xdai/interfaces/IAMB.sol";
+
+import "../interfaces/IHomeGateway.sol";
+import "../BaseHomeGateway.sol";
+
+contract xDaiGateway is BaseHomeGateway, xDaiL2Bridge {
+    constructor(
+        IArbitrator _arbitrator,
+        IForeignGateway _foreignGateway,
+        address _l1Target,
+        IAMB _amb
+    ) BaseHomeGateway(_arbitrator, _foreignGateway) xDaiL2Bridge(_l1Target, _amb) {}
+}