Feature Request: Add Flow#transform method?

Motivation:

Using an inline `transform` method can get more fluent code as `Flux`.
When using reactor-core 's `Flux`, we have something like:
```java
sink.asFlux()
.onBackpressureBuffer()
.transform(new MsgTransformer())
.subscribe(msgs -> {
... }
```

And the `transform` method is just a syntax sugar:
```java
public final Flux transform(Function, ? extends Publisher> transformer) {
if (Hooks.DETECT_CONTEXT_LOSS) {
transformer = new ContextTrackingFunctionWrapper(transformer);
}
return onAssembly(from(transformer.apply(this)));
}
```

WDYT?

Result:
More fluent code.