samples: net: lwm2m_client: set LWM2M_COAP_BLOCK_SIZE to 512
samples: net: lwm2m_client: set LWM2M_COAP_BLOCK_SIZE to 512

The native Zephyr LwM2M client enables the JSON formatter which
can use more of the packet buffers than the default 256 with
TLV formatting.  Let's set the default to 512 in order to avoid
cutting off the output of larger READ operations.

In the future, we should establish block transfer buffers to
handler longer READ ops.

Signed-off-by: Michael Scott <a17fed27eaa842282862ff7c1b9c8395a26ac320@foundries.io>

galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr