net: lwm2m: allow formatters to perform processing prior to read_op

Data formatters are becoming too complex for a simple do_read_op()
function to handle all in one place.  Also, more data formatters are
going to be added for LwM2M v1.1 support in the future.

In order for data formatters to perform internal setup or deny
invalid requests (specific to the formatter's logic), let's
establish do_read_op_* functions in each formatter.

Once the internal processing is done, they can call back into the
more generic lwm2m_perform_read_op function.

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