Feature Request: hook to get ResultSet before calling next

First things first - thanks for the amazing state of the art software.

I use it to stream large volume of data. It will be awesome to have a hook/callback called at the moment when ResultSet is just obtained. In particular I map ResultSet to CSV. It is all fine if result is not empty however I'd like return just a header (column names) when there the result is empty something like below.

```
public Flowable get(BiFunction onResultSetIsReady, BiFunction mapper)
```

Or may be extends ResultSetMapper?

And use it something like below
```
sb.select(sql).get(::toHeaderLine, ::toCsvLine): Flowable
```
Thanks one more time for the amazing software