Add Proxies from OpenZeppelin SDK (#2335)

contracts/mocks/DummyImplementation.sol

@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.6.0;
+
+abstract contract Impl {
+  function version() public pure virtual returns (string memory); 
+}
+
+contract DummyImplementation {
+  uint256 public value;
+  string public text;
+  uint256[] public values;
+
+  function initializeNonPayable() public {
+    value = 10;
+  }
+
+  function initializePayable() payable public {
+    value = 100;
+  }
+
+  function initializeNonPayable(uint256 _value) public {
+    value = _value;
+  }
+
+  function initializePayable(uint256 _value) payable public {
+    value = _value;
+  }
+
+  function initialize(uint256 _value, string memory _text, uint256[] memory _values) public {
+    value = _value;
+    text = _text;
+    values = _values;
+  }
+
+  function get() public pure returns (bool) {
+    return true;
+  }
+
+  function version() public pure virtual returns (string memory) {
+    return "V1";
+  }
+
+  function reverts() public pure {
+    require(false);
+  }
+}
+
+contract DummyImplementationV2 is DummyImplementation {
+  function migrate(uint256 newVal) payable public {
+    value = newVal;
+  }
+
+  function version() public pure override returns (string memory) {
+    return "V2";
+  }
+}