Transforms given cold ObservableSource into cold Flow.

The resulting flow is cold, which means that ObservableSource.subscribe is called every time a terminal operator is applied to the resulting flow.

A channel with the default buffer size is used. Use the buffer operator on the resulting flow to specify a user-defined value and to control what happens when data is produced faster than consumed, i.e. to control the back-pressure behavior. Check callbackFlow for more details.