net: lwm2m: Allow to acknowledge request early from the callback

LwM2M engine by default sends piggybacked responses for requests after
all callbacks are executed. This approach however isn't good enough if
the application callback executes some lenghty operations (for instance
during FW update). Delaying the ACK may result in unnecessary
retransmissions.

This commits adds an API function which allows to send an early empty
ACK from the application callback. This prevents further retransmissions
from the server side. After all callbacks are executed, the LwM2M engine
will send the response as a separate CON message.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>