samples: lwm2m: initial sample for LWM2M client

This sample utilizes the new LwM2M library by setting up default
values for LwM2M device and firmware objects and then establisting
a connection to a LwM2M server (for example Leshan Demo Server) via
the registration interface.

To use QEMU for this purpose please see:
doc/subsystems/networking/qemu_setup.rst

NOTE: This sample currently does not demonstrate DTLS/bootstrap as
neither of these is supported by the LwM2M library.

Signed-off-by: Michael Scott <michael.scott@linaro.org>