net: lwm2m: Add data validation callback

Add a data validation callback to the resource structure, which can be
registered by an application. It allows to verify the data before
actually modifying the resource data.

If the callback is registered for a resource, the data is decoded into a
temporary buffer first, and only copied into the actual resource buffer
if the validation is successfull. If no validation is required (and thus
no callback registered) the resource value is decoded directly into the
resource buffer, as it used to be.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>