fix: [NGOv2.x] Add OnPreShutdown to NetworkManager

[jonathanhertz]

Forwardport of #3358

This PR adds a NetworkManager.OnPreShutdown callback that happens before the internal shutdown is done. This is to allow accessing any state that is cleaned up by the NetworkManager during shutdown, such as accessing dynamically spawned NetworkObjects.

Changelog

• Added: The event NetworkManager.OnPreShutdown has been added which is called before the NetworkManager cleans up and shuts down.

Testing and Documentation

• A test has been added to ensure this is being called, and called before OnServerStopped.

[NoelStephensUnity]

👍