Provide a way to force a refresh of the cached ToOne target

When the target entity of a ToOne relationship is changed after it's resolved and cached, there is no way to force the target to refresh from the database.

Describe the solution you'd like
Provide a function that can be called that forces a new "get" from the box, caches the new entity, and returns it. This can be used when the app knows that the entity has been updated but wants those changes reflected in the related object.

Describe alternatives you've considered
As a workaround/alternative, the app can find the correct box, get an updated entity using the getTargetId(), then set the ToOne target entity to the updated entity. This works but requires a lot of extra code for something it feels like objectbox should be able to handle (especially since the ToOne relationship already knows the correct box for the get operation).

Additional context
Currently using this on Kotlin/Android.

首先定位 ToOne 关系 API，并阅读现有的 getTargetId() 和 target setter 行为。实现请求的 refresh 操作，使其从已知的 box 中获取相关实体，更新缓存并返回该实体；完成的标准是调用方可以刷新已更改的 target，而无需手动查找 box。