Kotlin: add flow extension functions for BoxStore and Query

[greenrobot-team]

Proposal to close #807

This will allow syntax like, for example to create a flow to listen to all changes to a box:

val flow = store.subscribe(TestEntity::class.java).toFlow()

Or to get the latest query results on any changes to a box:

val flow = box.query().subscribe().toFlow()

Also adds shortcuts if the subscription does not need to be customized:

val flow = store.flow(TestEntity::class.java)
val queryFlow = query.flow()