Add _initializableStorageSlot (#5337)

Co-authored-by: Arr00 <13561405+arr00@users.noreply.github.com>

.changeset/fast-coats-try.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`Initializable`: Add `_initializableStorageSlot` function that returns a pointer to the storage struct. The function allows customizing with a custom storage slot with an `override`.

contracts/proxy/utils/Initializable.sol

@@ -216,13 +216,23 @@ abstract contract Initializable {
         return _getInitializableStorage()._initializing;
     }
 
+    /**
+     * @dev Pointer to storage slot. Allows integrators to override it with a custom storage location.
+     *
+     * NOTE: Consider following the ERC-7201 formula to derive storage locations.
+     */
+    function _initializableStorageSlot() internal pure virtual returns (bytes32) {
+        return INITIALIZABLE_STORAGE;
+    }
+
     /**
      * @dev Returns a pointer to the storage namespace.
      */
     // solhint-disable-next-line var-name-mixedcase
     function _getInitializableStorage() private pure returns (InitializableStorage storage $) {
+        bytes32 slot = _initializableStorageSlot();
         assembly {
-            $.slot := INITIALIZABLE_STORAGE
+            $.slot := slot
         }
     }
 }