net: l2: openthread: Implement `enable` API function

It fixes #issues/26220.

openthread_start function is called when L2 enable(iface, true) is
called. openthread_stop is called when L2 enable(iface, false) is
called. openthread_stop makes the device to leave the OpenThread
network.

Signed-off-by: Lukasz Maciejonczyk <lukasz.maciejonczyk@nordicsemi.no>