[radio] add OT_RADIO_CAPS_RX_ON_WHEN_IDLE capability (#9554) (#9554)

Add a new `OT_RADIO_CAPS_RX_ON_WHEN_IDLE` radio capability which lets
OpenThread know what the radio idle state operation will be: receive
or sleep.

This allows to save power on sleepy devices since the active --> idle
transition is much faster, specially for the cases in which there is
some kind of serialization between OpenThread core and the radio
driver.

tests/unit/test_platform.cpp

@@ -126,6 +126,8 @@ OT_TOOL_WEAK void otPlatRadioSetShortAddress(otInstance *, uint16_t) {}
 
 OT_TOOL_WEAK void otPlatRadioSetPromiscuous(otInstance *, bool) {}
 
+OT_TOOL_WEAK void otPlatRadioSetRxOnWhenIdle(otInstance *, bool) {}
+
 OT_TOOL_WEAK bool otPlatRadioIsEnabled(otInstance *) { return true; }
 
 OT_TOOL_WEAK otError otPlatRadioEnable(otInstance *) { return OT_ERROR_NONE; }