Add campaign property for campaign start time.

This field is optional. Omitting the field (or using only a date instead
of a full timestamp) will use the old behavior of picking a random
daylight hour to start the campaign.

This doesn't include any UI in the new game wizard yet. This is only a
campaign yaml option.

https://github.com/dcs-liberation/dcs_liberation/issues/2400

qt_ui/main.py

@@ -286,6 +286,7 @@ def create_game(
         ),
         GeneratorSettings(
             start_date=start_date,
+            start_time=campaign.recommended_start_time,
             player_budget=DEFAULT_BUDGET,
             enemy_budget=DEFAULT_BUDGET,
             inverted=inverted,