net: lwm2m: fix OPAQUE handling in LwM2M engine

With the change to support multi-fragement buffers in the LwM2M subsys,
the OPAQUE data type was direct write methods were broken.

Let's fix OPAQUE handling by using the newly introduced getter methods
which can use multiple user callbacks (depending on the size of the
user provided buffer).  Let's also add public methods for users to set
/ get OPAQUE data in resources for future use with DTLS key data.

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