net: lwm2m: add gateway callback to handle prefixed messages

Adding a callback for handling lwm2m messages with prefixed paths defined
by the gateway object. If CONFIG_LWM2M_GATEWAY_OBJ_SUPPORT is set,
each path is checked for the prefix stored in the object instances of the
gateway object 25. If prefixes match the msg is passed to the gw_msg_cb.

Signed-off-by: Simon Walz <simon.walz@autosen.com>