net: lwm2m: Added execute arguments support

A dedicated LwM2M execute callback type has been implemented which
supports execute arguments. The lwm2m engine, lwm2m_client sample and
lwm2m objects have been updated accordingly. Also the API change has
been documented, and the lwm2m engine reference has been updated.

Fixes #30551.

Signed-off-by: Maik Vermeulen <maik.vermeulen@innotractor.com>