net: openthread: add wake-up coordinator support

Add Kconfig option `OPENTHREAD_WAKEUP_COORDINATOR` to enable
the Wake-up Coordinator role. Implement API to set Coordinated
Sampled Transmitting sample time and period for a driver that supports
`IEE802154_OPENTHREAD_HW_CST` capability.

The feature is be enabled on with ieee802154_nrf5 driver with option
by setting default value of CONFIG_IEEE802154_NRF5_CST_ENDPOINT.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>
Co-authored-by: Andrzej Kuroś <andrzej.kuros@nordicsemi.no>