Add capability to execute a function on each scan period execution

[StephenOTT]

The retriever.listen function is only called when a change is detected.

Would be great if we could add the ability to configure code to execute when the scan period executes.

[cescoffier]

Do you mean before or after the scan?

[StephenOTT]

In my case it was a before scan. The use case was loggin: I wanted to log every time a scan occurs

[cescoffier]

I have added:

@Fluent
ConfigRetriever setBeforeScanHandler(Handler<Void> function);

[StephenOTT]

@cescoffier if a After is possible, that would be good as well. Could see wanting to only log on successful pull of the data

[cescoffier]

@StephenOTT I've have also added:

ConfigRetriever setConfigurationProcessor(Function<JsonObject, JsonObject> processor);

The passed function is called after the scan and allow validating the configuration, or even update it.