net: lwm2m: Add observe callback for observe and notification events

Added an observe callback so that the application can register to
receive events like observer added/deleted, and notification acked/
timed out. The notifications can be traced back to the exact data
contained within them by use of the user_data pointer.

Fixes #38531.

Signed-off-by: Maik Vermeulen <maik.vermeulen@innotractor.com>