net: lwm2m: Added LwM2M Send operation support

LwM2M engine Send operation support /dp.
Send operation based on Composite Read functionality
which is not enabled at engine side like composite write.
Added Kconfig configurable for composite read path list size.
Created generic Read object instance which is shared between
general read and Composite Read operation.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>