net: lwm2m: add optional timestamp resources to some IPSO objects

Based on work by Michael Scott.

Add a new Kconfig knob, CONFIG_LWM2M_IPSO_TIMESTAMP_EXTENSIONS. This
defaults to n. When enabled, various IPSO objects will by default have
the timestamp resource (5518) added to their representations. This can
be turned off on a per-object basis.

The idea of adding timestamp resources was originally suggested by
Hannes Tschofenig on this OMA page:

https://github.com/OpenMobileAlliance/OMA_LwM2M_for_Developers/issues/429



Signed-off-by: Michael Scott <mike@foundries.io>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>