on_rocket_constructed event

[Quezler]

Currently we have the `on_rocket_launch_ordered` event for when a rocket launches either via the cargo checkbox or a manual button press,
however there seems to be no exposed event for when a rocket has been constructed. (100% progress on the parts, silo starts blinking & opens)

For one of my mods i am already depending on the `on_rocket_launch_ordered` event in order to do something the tick the silo doors close again after a launch:
https://mods.factorio.com/mod/se-cargo- ... ion-helper

For that purpose i have written a small mod that fires a custom `on_rocket_silo_status_changed `after a rocket launch has been ordered:
https://mods.factorio.com/mod/glutenfre ... ilo-events (times the first rocket launch & caches it for subsequent launches)

However for my next mod project i could use a reliable & low ups way to get the tick the silo starts going through its pre-launch status stages,
hence the request for a `on_rocket_constructed` event for when the status changes from building_rocket to create_rocket.

The alternative seems to be checking `force.item_production_statistics` or the statuses of all the rocket silos every second or so, which is not ideal. ^-^