Reorganize the repo structure (#2503)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>

contracts/proxy/beacon/IBeacon.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev This is the interface that {BeaconProxy} expects of its beacon.
+ */
+interface IBeacon {
+    /**
+     * @dev Must return an address that can be used as a delegate call target.
+     *
+     * {BeaconProxy} will check that this address is a contract.
+     */
+    function implementation() external view returns (address);
+}