Add support for mapping "view" before saving to DB for materialized views

This is useful if e.g. you have a stream version outside the event. For example:

```kotlin
val event = ...
val streamVersion = 123L // Out of band

materializedSearchView.update(event) { viewBeforeSave ->
     viewBeforeSave.copy(streamVersion = streamVersion)
}
```

i.e. add a function that maps from view V to V that allows you to perform "changes" like descriped above.