net: lwm2m: Rework bootstrap DELETE operation

Rework the bootstrap DELETE operation, to support deletion of multiple
resources.

Current implementation had several oversimplifications, making it not
spec-compliant:
* DELETE `/` removed only Security object instances (!= 0)
* DELETE `/x` was handled as DELETE `/x/0`, therefore not removing all
  of the object instances.

Since the above is only supported during bootstrap and not regular
Device management, this functionality was implemented in the
`bootstrap_delete` function, which now will be called for all DELETE
operations initiated during bootstrap. The regular LwM2M DELETE handler
will only be called during regular Device management, as it has more
strict limitations on what can be deleted.

Additionally, handle empty URI Path option as `/`, therefore indicating
deletion of all resources.

Fixes #29964

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>