openthread: Add Kconfigs for packet TX time and carrier functions

Add new Kconfig options to enable platform-specific features in
OpenThread:

- CONFIG_OPENTHREAD_PLATFORM_PKT_TXTIME enables support for packet TX
time when CONFIG_NET_PKT_TXTIME is selected.
- CONFIG_OPENTHREAD_PLATFORM_CARRIER_FUNCTIONS enables modulated and
continuous carrier functions when CONFIG_OPENTHREAD_DIAG and
CONFIG_IEEE802154_CARRIER_FUNCTIONS are enabled.

Update preprocessor conditionals to use the new options instead of
relying directly on CONFIG_NET_PKT_TXTIME and
CONFIG_IEEE802154_CARRIER_FUNCTIONS.

This change improves configurability and allows to reuse the platform
implementation when not using Zephyr networking.

Signed-off-by: Adrian Gielniewski <adrian.gielniewski@nordicsemi.no>