net: openthread: Use settings subsystem

OpenThread settings implementation built on top of Zepyhr settings
submodule.

With this solution, OpenThread settings are identified with keys of
the following format: `ot/id/instance`, where `id` is assigned by
OpenThread stack, and `instance` is a 32-bit random number, both in
hex. The implementation makes use of `settings_load_subtree_direct`
function to iterate over settings instances. This allows the
OpenThread settings layer to be a fully transparent shim layer between
OpenThread/Zephyr APIs.

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