feat: optional dontdestroyonload for networkscenemanager

related to #587.

Without cutting any deeper I am ok with adding the option but defaulting it to enabled for now. I still think the current behavior is what most people would use. If not its a simple check box to change it :)

!!!NOTE: If you disable the DDOL setting for NetworkSceneManager and do a Normal (non additive) scene change you will need to take extra steps to ensure your Network containing GameObject is not destroyed in that operation!!!

--- Original GitHub issue (auto-fetched) ---
Title: feat: optional dontdestroyonload for networkscenemanager

Body:
related to #587.

Without cutting any deeper I am ok with adding the option but defaulting it to enabled for now. I still think the current behavior is what most people would use. If not its a simple check box to change it :)

!!!NOTE: If you disable the DDOL setting for NetworkSceneManager and do a Normal (non additive) scene change you will need to take extra steps to ensure your Network containing GameObject is not destroyed in that operation!!!
