SubscriptionRequest(entityTypenon-null, clientnon-null, actorRequestFactorynon-null)
A request to subscribe to updates of entity states of a certain type.
Allows to obtain the `EntitySubscriptionObject` which exposes the entity changes in a form of
callbacks which can be subscribed to.
A usage example:
```
client.subscribeTo(Task.class)
.where(Filters.eq("status", Task.Status.ACTIVE))
// Additional filtering can be done here.
.post()
.then(({itemAdded, itemChanged, itemRemoved, unsubscribe}) => {
itemAdded.subscribe(_addDisplayedTask);
itemChanged.subscribe(_changeDisplayedTask);
itemRemoved.subscribe(_removeDisplayedTask);
});
```
If the entity matched the subscription criteria at one point, but stopped to do so, the
`itemRemoved` callback will be triggered for it. The callback will contain the last entity state
that matched the subscription.
Please note that the subscription object should be manually unsubscribed when it's no longer
needed to receive the updates. This can be done with the help of `unsubscribe` callback.