feat: upgrade governance strategy

- governance strategy plugin can be updated by successful proposal.

contracts/governance/interfaces/governance/IGovernanceEvents.sol

@@ -42,6 +42,10 @@ interface IGovernanceEvents {
     /// @param proposalId Number of the proposal.
     event ProposalExecuted(uint256 proposalId);
 
+    /// @notice Emmited when the governance strategy is updated.
+    /// @param newStrategy Address of the new strategy contract.
+    event StrategyUpdated(address newStrategy);
+
     /// @notice Emitted when voting thresholds get updated.
     /// @dev Only governance can update thresholds.
     /// @param proposalThreshold Number of votes required to add a proposal.

contracts/governance/interfaces/governance/IGovernanceUpgrade.sol

@@ -20,15 +20,20 @@
 pragma solidity >=0.8.0 <0.9.0;
 
 interface IGovernanceUpgrade {
-    /// @notice Updates the governance implementation address.
-    /// @dev Only callable after successful voting.
-    /// @param newImplementation Address of the new governance implementation contract.
-    function upgradeImplementation(address newImplementation) external;
-
     /// @notice Updates the proposal and quorum thresholds to the given values.
     /// @dev Only callable by the governance contract itself.
     /// @dev Thresholds can only be updated via a successful governance proposal.
     /// @param newProposalThreshold The new value for the proposal threshold.
     /// @param newQuorumThreshold The new value for the quorum threshold.
     function updateThresholds(uint256 newProposalThreshold, uint256 newQuorumThreshold) external;
+
+    /// @notice Updates the governance strategy plugin.
+    /// @dev Only callable by the governance contract itself.
+    /// @param newStrategy Address of the new strategy contract.
+    function updateGovernanceStrategy(address newStrategy) external;
+
+    /// @notice Updates the governance implementation address.
+    /// @dev Only callable after successful voting.
+    /// @param newImplementation Address of the new governance implementation contract.
+    function upgradeImplementation(address newImplementation) external;
 }

contracts/governance/mixins/MixinUpgrade.sol

@@ -20,6 +20,7 @@
 pragma solidity >=0.8.0 <0.9.0;
 
 import "../../utils/storageSlot/StorageSlot.sol";
+import "../interfaces/IGovernanceStrategy.sol";
 import "./MixinStorage.sol"; // storage inherits from interface which declares events
 
 abstract contract MixinUpgrade is MixinStorage {
@@ -58,11 +59,21 @@ abstract contract MixinUpgrade is MixinStorage {
         uint256 newProposalThreshold,
         uint256 newQuorumThreshold
     ) external override onlyDelegatecall onlyGovernance {
+        IGovernanceStrategy(_governanceStrategy().value).assertValidThresholds(newProposalThreshold, newQuorumThreshold);
         _governanceParameters().proposalThreshold = newProposalThreshold;
         _governanceParameters().quorumThreshold = newQuorumThreshold;
         emit ThresholdsUpdated(newProposalThreshold, newQuorumThreshold);
     }
 
+    /// @inheritdoc IGovernanceUpgrade
+    function updateGovernanceStrategy(address newStrategy) external override onlyDelegatecall onlyGovernance  {
+        address oldStrategy = _governanceStrategy().value;
+        assert(newStrategy != oldStrategy);
+        require(_isContract(newStrategy), "UPGRADE_NOT_CONTRACT_ERROR");
+        _governanceStrategy().value = newStrategy;
+        emit StrategyUpdated(newStrategy);
+    }
+
     /// @dev Returns whether an address is a contract.
     /// @return Bool target address has code.
     function _isContract(address target) private view returns (bool) {