Support data observers for related entities

Note these related feature requests:

• #126
• #268

This might be a feature request.
It seems that currently it is not possible to listen for nested changes.

• ObjectBox version: 1.2.1
• Reproducibility: always

boxFor(Item::class.java).query().build().subscribe().observer { items ->
    // Not called when Note.text changes.
}

Entities

@Entity
class Item {

    @Id
    open var id: Long = 0L

    lateinit open var notes: ToMany<Note>
}

@Entity
class Note {

    @Id
    open var id: Long = 0L

    open var text: String? = null

    @Backlink
    lateinit open var parentItem: ToOne<Item>
}

首先复现使用 Item 和 Note 实体的 Kotlin 示例，重点关注 Note.text 发生变化时的查询订阅和 observer 行为。确定相关实体预期的通知行为，并验证嵌套更新 Note 后 Item observer 会收到通知；该 issue 未指明源文件或测试。