net: lwm2m: introduce output context user_data
net: lwm2m: introduce output context user_data

Data formatters have various private state variables which are
currently located in the output context structure.  Let's add
a place where data formatters can store a pointer to their
private data so that as we add more formatters the output
context doesn't get cluttered up.

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

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