Make transfer-04 show how `callbacks` work

# Feature Request

Currently sample transfer-04 shows how to implement a custom listener, but that's a bit low level detail, the sample should show how the `callbacks` mechanism is working:
ref. https://eclipse-edc.github.io/documentation/for-contributors/runtime/service-layers/#6-events-and-callbacks

## Which Areas Would Be Affected?

_e.g., DPF, CI, build, transfer, etc._

## Why Is the Feature Desired?

_Are there any requirements?_

## Who will sponsor this feature?

_Please @-mention the committer that will sponsor your feature_.

## Solution Proposal

_If possible, provide a (brief!) solution proposal._