net: lwm2m: rd_client_stop behavior

[SebastianSchueller]

Since #76470 the RD client will emit a DISCONNECT essentially every time the client enters the ENGINE_DEREGISTERED state.
One consequence of this changes is that calling rd_client_stop on the RD client when its already "shut down" (i.e. in ENGINE_IDLE state) will cause the client to emit DISCONNECT.
While this wasn't the original intent of the PR, I think this feature is really nice. It makes it really easy and robust for applications to check if the RD client is still doing something by calling rd_client_stop until a DISCONNECT event is fired.

I'd really like to use this as a feature in our application, but I don't want to create a spacebar-heater scenario here :)
Is there any interest in adding this functionality to the documented / tested interface of the RD client?

I'd be happy to create a PR for the tests and documentation for it.