net: lwm2m: SenML-CBOR Encoder and CDDL update

Added support for historical data encode by adding base time (bt)
and time (t) label. New labels are needed for Encoder so Decoder
is not regenerated.

Added support for SenML-CBOR to write time series data.
Use "bt" base time and "t" timestamp labels for data cache.

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