Rotate by pivot inside Unity editor

RotationPivot.cs

using UnityEngine;

public class RotationPivot: MonoBehaviour
{
    /**
     * The transform of the object you want to rotate with this pivot 
     */
    public Transform target;

    
    public void SetRotation(Quaternion newRotation)
    {
        //We store current parents of pivot and target
        Transform oldPivotParent = transform.parent;
        Transform oldTargetParent = target.transform.parent;

        RemoveParents();

        //Set the pivot as the parent of target
        target.transform.SetParent(transform);

        //Apply new rotation to pivot. Target follows since it has the pivot as parent now
        transform.rotation = newRotation;

        RemoveParents();

        transform.SetParent(oldPivotParent);
        target.transform.SetParent(oldTargetParent);
    }

    /*
    * Reset parent of target and pivot. We need this to make both switchable
    */
    private void RemoveParents()
    {
        target.transform.SetParent(null);
        transform.SetParent(null);
    }
}

RotationPivotEditor.cs

using UnityEditor;
using UnityEngine;

/**
 * Custom editor for RotationPivot 
 */
[CustomEditor(typeof(RotationPivot))]
public class RotationPivotEditor : Editor {

    /**
     * Gets called everytime the scene GUI is redrawn. Only if the gameObject is slected in the inspector
     */
    void OnSceneGUI()
    {
        //The RotationPivot component of the selected game object
        RotationPivot rotationPivot = (RotationPivot)target;

        //Return if rotationPivot has no target
        if(rotationPivot.target == null)
        {
            return;
        }

        //Display a RotationHandle with the position and rotation of rotationPivot and store its value inside newRotation
        Quaternion newRoation = Handles.RotationHandle(
            rotationPivot.transform.rotation, 
            rotationPivot.transform.position
        );

        //Continue only if the new rotation differs from the current rotation
        if (newRoation != rotationPivot.transform.rotation)
        {
            //Register a undo action both on the rotationPivot and its target
            Undo.RecordObject(rotationPivot.transform, "Rotate by pivot");
            Undo.RecordObject(rotationPivot.target, "Rotate by pivot");

            //Apply the new rotation
            rotationPivot.SetRotation(newRoation);
        }
    }
}