Add the aRAI rate provider

contracts/aRAIRedemptionRateProvider.sol

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+// SPDX-License-Identifier: GPL-3.0-or-later
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+pragma solidity ^0.8.0;
+
+import "./interfaces/IRateProvider.sol";
+import "./interfaces/IRedemptionPriceSnap.sol";
+import "./interfaces/IAaveV2LendingPool.sol";
+
+/**
+ * @title RAI Redemption Price Rate Provider
+ * @notice Returns the redemption price of RAI in terms of USD
+ */
+contract RAIRedemptionRateProvider is IRateProvider {
+    address public immutable rai;
+    IRedemptionPriceSnap public immutable redemptionPriceSnap;
+    IAaveV2LendingPool public immutable aaveV2LendingPool;
+
+    constructor(
+        address _rai,
+        IRedemptionPriceSnap _redemptionPriceSnap,
+        IAaveV2LendingPool _aaveV2LendingPool
+    ) {
+        rai = _rai;
+        redemptionPriceSnap = _redemptionPriceSnap;
+        aaveV2LendingPool = _aaveV2LendingPool;
+    }
+
+    /**
+     * @return The redemption price of RAI in terms of USD
+     */
+    function getRate() external view override returns (uint256) {
+        // snappedRedemptionPrice is RAY, we need to convert it to WAD
+        uint256 raiRedemptionPrice = redemptionPriceSnap.snappedRedemptionPrice() / 10**9;
+
+        // Return the RAI aToken USD price as WAD
+        return (raiRedemptionPrice * aaveV2LendingPool.getReserveNormalizedIncome(rai)) / 10**27;
+    }
+}

contracts/interfaces/IAaveV2LendingPool.sol

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+// SPDX-License-Identifier: GPL-3.0-or-later
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+pragma solidity ^0.8.0;
+
+interface IAaveV2LendingPool {
+    function getReserveNormalizedIncome(address asset) external view returns (uint256);
+}

contracts/interfaces/IRedemptionPriceSnap.sol

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+// SPDX-License-Identifier: GPL-3.0-or-later
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+pragma solidity ^0.8.0;
+
+interface IRedemptionPriceSnap {
+    function snappedRedemptionPrice() external view returns (uint256);
+}