net: lwm2m: Implement gt/lt/st attributes handling

While it was possible to specify Greater Than / Less Than / Step
observe attributes for a resource, they were not taken into
consideration when evaluating notification criteria. This commit adds
support for checking if the resource value meets the value criteria
specified by gt/lt/st attributes.

Note that gt/lt attributes are thresholds - notification should only
sent if the resource value crosses them.

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