net: openthread: add capability IEEE802154_OPENTHREAD_HW_CST

The new capability `IEEE802154_OPENTHREAD_HW_CST` is added.
The option MAY be implemented as an extension to IEEE 802.15.4
drivers allowing to inject CST related Information Elements.

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