[plugwise] Use ThingHandlerService for discovery (#9041)

[plugwise] Use ThingHandlerService for discovery (#9041)

This simplifies the PlugwiseHandlerFactory code so it no longer needs to register and keep track of a discovery service for each bridge.

Also contains a few other improvements:

* fix lastRequest timestamp not updated when sending messages to discover node properties
* use java.time Duration and Instant
* use List.of, Set.of
* remove redundant null suppression because of EEAs

Signed-off-by: Wouter Born <github@maindrain.net>