net: lwm2m: introduce input formatter private data
net: lwm2m: introduce input formatter private data

The JSON formatter is currently not enabled for incoming WRITE
operations.  To update the code in the formatter and not litter
the input context with extra data, let's allow formatters to
store their own user data.

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

explora26/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr