[REST] Issue an event when configurations have changed

[spacemanspiff2007]

With oh4 it's possible to read the available transformations.
However openHAB does not sent an event when a transformation changes making caching the transformations very hard.
It would be nice if an event would be sent if any transformation changes (e.g. TransformationChangedEvent).

Imho this event doesn't have to have the fine granularity of the item events - a generic ConfigurationUpdatedEvent would also be enough which would contain "transformations".
That way it could be reused for e.g. thing actions or e.g. #3611.

[florian-h05]

Implementing caching for transformations should actually be really simple, just implement a RegistryChangeListener for the TransformationRegistry (like it’s done for the other registries in #3335). But for external systems, it would be helpful to have such events.

[spacemanspiff2007]

An event for #3729 would also be nice