Add Support for Nested NetworkObjects in Prefabs

Is your feature request related to a problem? Please describe.
One major pain point of using Unity Netcode for GameObjects (as well as many other networking solutions) is the lack of support for spawning nested NetworkObjects. Currently, if you try to spawn a prefab with nested NetworkObjects, the primary NetworkObject will be networked, while all the child NetworkObjects will not.

As an example, imagine that you are trying to spawn a boat with cannons that each player can move and shoot. In this scenario, it would make sense that (1) cannons should be parented to the boat and (2) any player should be able to take ownership of a cannon (separate of the boat) when they go to move/shoot it. This means that both the boat and the cannons would need to be NetworkObjects. However, because of this limitation, you would not be able to make one boat prefab with the cannons on it.

Describe the solution you'd like
Ideally, when a prefab with nested NetworkObjects is spawned, it should automatically network any child network objects.

Describe alternatives you've considered
You can work around the issues with nested NetworkObjects, but it requires designing your prefabs/scripts in less intuitive ways.

For instance, in the boat example I mentioned earlier, you could create cannon spawners that spawn the cannons separately when the boat is spawned. However, this has its own issues. Primarily:

1. It is harder to position the cannons.
   • If you just try to position the cannons with the transform of the spawner, it will be difficult to visualize the final product.
   • Positioning the cannon prefab first and then replacing it with the spawner requires more work than necessary. Also, what if the boat model changes and you need to reposition the cannons? You'll have to add back in the cannon prefab, position it, move the spawner to that position, and delete the cannon prefab.
2. In Editor, scripts on the boat cannot reference scripts on the cannons (and vice versa).
   • This means you may have to wire up scripts at runtime.
   • This also means that your code will have to check to make sure that the cannons have been spawned in before doing anything with them, since they aren't necessarily spawned at the same time as the boat.

There are other solutions to these types of problems, but they are generally more difficult to implement and/or encourage poor design.

Additional context
N/A

未指定文件或测试。首先跟踪 prefabs 和 NetworkObjects 是如何生成的，然后确定嵌套子对象应如何注册和拥有；完成的标准是，生成一个 prefab 会自动将其嵌套的 NetworkObjects 联网，同时保留父子关系和独立的所有权。