net: lwm2m: introduce put_begin/end for object instance and resources

Data formatters may need to process data at the beginning and end of
each object instance and/or resource.  Currently, they can only add
processing at the beginning and end of resource instances.

Let's establish put_begin/end_oi (object instance) and put_begin/end_r
(resource) API functions that data formatters can use for this purpose.

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