EventSubscriptionRequest(eventTypenon-null, clientnon-null, actorRequestFactorynon-null)
A request to subscribe to events of a certain type.
Allows to obtain the `EventSubscriptionObject` which reflects the events that happened in the
system and match the subscription criteria.
A usage example:
```
client.subscribeToEvent(TaskCreated.class)
.where([Filters.eq("task_priority", Task.Priority.HIGH),
Filters.eq("context.past_message.actor_context.actor", userId)])
.post()
.then(({eventEmitted, unsubscribe}) => {
eventEmitted.subscribe(_logEvent);
});
```
The fields specified to the `where` filters should either be a part of the event message or
have a `context.` prefix and address one of the fields of the `EventContext` type.
The `eventEmitted` observable reflects all events that occurred in the system and match the
subscription criteria, in a form of `spine.core.Event`.
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.