Support of RxJava

[GrishinSergey]

I know about this thread #326, but is is preety old and not updated, as I see, so I made new one.
So, some use cases of RxJava & Exposed:

1. Data observability.
   Will be cool to subscribe to data, and, when something changed in table, receive update via rx observable. It is really useful, for example, in android apps. If this feature will be supported, I guess, that this will be the reasin to use Exposed instead other ORMs in android.
2. Threads managing.
   As for me, will be cool, to manage threads for database, like in RxJava (subscribeOn , observeOn) (ok, it's also possible via coroutines)
3. Flowable's back-pressure.
   Returning to examples in android ecosystem, imagine case, when database used as cache. UI subscribed to it and will be notified, when data were updated. And, just imagine, that this data size is huge, so, it will be usefull to handle via Flowable's back-pressure.

[nikolamin]

It will be very useful to have Exposed on the server and the Android app, especially when you have the same schema on the backend and in the apps. But, not having this feature is a deal-breaker for choosing Exposed over RoomDatabase on Android.