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>
}

Start by reproducing the Kotlin example with Item and Note entities, focusing on the query subscription and observer behavior when Note.text changes. Determine the intended notification behavior for related entities and verify that the Item observer is notified after a nested Note update; the issue names no source files or tests.