net: lwm2m: rework resource instance storage / access methods

LwM2M allows for multiple instance resources such the power source
resources in the device object.  These types of resources have
always been very hard to work with, and frankly were poorly
implemented.

This led to other issues where it was very hard to have
non-sequential resource instances, and each resource of this type
needed special getter / setter methods such as:
lwm2m_device_add_pwrsrc()
lwm2m_device_set_pwrsrc_voltage_mv()

Going forward, as more LwM2M objects are implemented this just
doesn't scale well.

To fix this:
- split the resource instance data out from the resource data.
  This includes the data pointer information and resource
  instance id.
- add resource id and resource instance id to the event callback
  functions so user's can see in more detail what resources and
  resource instances are being handled.
- allow generic functions like lwm2m_engine_get_*() and
  lwm2m_engine_set_*() to access resource instance data.
- adjust object resource initialization  macros to map resource
  instances to resources at the time of object instance
  creation.
- fix up the lwm2m_client as a reflection of all of these changes.

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